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_DECLASSIFTED 
authority Secretary of 


SEP 7 1960 

Defense memo 2 August 1960 


LIBRARY OF CONGRESS 


SUMMARY TECHNICAL REPORT 
OE THE 

NATIONAL DEEENSE RESEARCH COMMITTEE 



1 liis docuineiU contains infonnaLion aliccting tlic national defc'nse of the 
United States within the nieaninf> of the Espionage Act, 50 U. S. C., 31 and 32, 
as amended. Its transmission or the revelation of its contents in any manner to 
an unauthorized person is prohibited by law. 

This volume is cla.ssified S o t R If I I in accordance with security regula- 

hapters contain mate- 



Other chapters may 


have had a lower classification or none. The reader is advised to consult the 
War and Navy agencies listed on the reverse of this page for the current 
classification of any material. 











Manuscript and illustrations for this volume were prepared for 
publication by the Summary Reports Group of the Columbia Uni¬ 
versity Di\ ision of War Research under contract OEMsr-1131 with 
the Office of Scientific Research and Development. This volume 
was printed and bound by the Columbia University Press. 

Distribution of the Summary Technical Report of XDRC has been 
made by the \Var and Navy Departments. Inquiries concerning the 
availability and distribution of the Summary Technical Report 
volumes and microfilmed and other reference material should be 
addressed to the War Department Library, Room lA-522, The 
Pentagon, Washington 25, D. C., or to the Office of Naval Research, 
Navy Department, Attention: Reports and Documents Section, 
W'ashington 25, D. C. 

'll.) 

Copy No. 

" 239 

Phis volume, like the seventy others of the Summary Technical 
Report of NDRC, has been written, edited, and printed under 
great pressure. Inevitaldy there are errors which have slipped past 
Di\ ision readers and proofreaders. There may be errors of fact not 
known at time of printing. The author has not been able to follow 
through his writing to the final page proof. 

Please report errors to: 

JOINT RESEARCH AND DEVELOPMENT BOARD 
PROGRAMS DIVISION (STR ERRATA) 

WASHINGTON 25, D. C. 

.\ master errata sheet will be compiled from these reports and sent 
to recipients of the volume. Your help will make this book more 
useful to other readers and will be of gi eat value in preparing any 
revisions. 





SUMMARY TECHNICAL REPORT OE DIVISION 6. NDRC 


VOLUME 6A 


declassified 

By authority Secretary of 


THE APPLICATION' ™ 

Defense memo 2 August 1960 

OF OCEANOGRAPHT'TO'™'^ 


SUBSURFACE WARFARE 


LC REGULA TION: BEFORE SERVICING 
OR REPRODUCIISG ANY PART OF THIS 
document, ALL CLASSIFICATION 
TjjAPTTTNGS MUST BE CANCELI^K 


OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT 
VANNEVAR BUSH, DIRECTOR 


NATIONAL DEFENSE RESEARCH COMMITTEE 
JAMES B. CONANT, CHAIRMAN 


DIVISION 6 

JOHN T. TATE, CHIEF 


WASHINGTON, D. C., 1946 









NATIONAL DEFENSE RESEARCH COMMITTEE 


James B. Conant, Chairvian 
Richard C.Tolman, Vice Chairman 
Roger Adams Army Represcnlativci 

Frank B. Jewett Navy Rcprescntativc- 

Karl T. Compton Commissioner of Patents^ 

Irvin Stewart, Executive Secretary 


*Army representatives in order of service: 
Maj. Gen. G. V. Strong Col. L. A. Denson 

Maj. Gen. R. C. Moore Col. P. R. Faymonville 

Maj. Gen. C. C. ^Villiams Brig. Gen. E. A. Regnier 
Brig. Gen. W. A. Wood, Jr. Col. M. M. Irvine 
Col. E. A. Routheau 


-Naxiy representatwes in order of sen/ice: 

Rear Adin. II. G. Bowen Rear Adni. J. .\. Purer 

Capt. Lybrand P. Smith Rear .\dm. H. \ an Keuren 

Commodore H. A. Schade 
^Cornxnissioners of Patents in order of service: 
Contvay P. Coe Casper \V. Ooms 


NOTES ON THE ORGANIZATION OF NDRC 


Phe duties of the National Defense Research Committee 
were (1) to recommend to the Director of OSRD suitable 
projects and research programs on the instrumentalities of 
warfare, together with contract facilities for carrying out 
these projects and programs, and (2) to administer the tech¬ 
nical and scientific work of tlie contracts. More specifically, 
NDRC functioned by initiating research projects on re¬ 
quests from the .\rmy or the Navy, or on requests from an 
allied government transmitted through the Liaison Office 
of OSRD, or on its own considered initiative as a result of 
the experience of its members. Proposals prepared by the 
Division, Panel, or Committee for research contracts for 
performance of the ^vork in\olved in such projects tvere 
first reviewed by NDRC, and if approved, recommended to 
the Director of OSRD. Upon approval of a proposal by the 
Director, a contract permitting maximum flexibility of 
scientific effort was arranged. The business aspects of the 
contract, including such matters as materials, clearances, 
vouchers, patents, priorities, legal matters, and administra¬ 
tion of patent matters were handled by the Executive Sec¬ 
retary of OSRD. 

Originally NDRC administered its work through five 
divisions, each headed by one of the NDRC members. 
These were: 

Division A —Armor and Ordnance 
Division B —Bombs, Euels, Gases, &: Chemical Problems 
Division C — Communication and Transportation 
Division D —Detection, Controls, and Instruments 
Division E — Patents and Inventions 


In a reorganization in the lall of 1942, ttventy-three ad¬ 
ministrative divisions, panels, or committees were created, 
each with a chief selected on the basis of his outstanding 
work in the particular field. The NDRC members then be¬ 
came a reviewing and advisory group to the Director of 
OSRD. The final organization was as follows: 


Div 

Div 

Div 

Di\ 

Div 

Div 

Div 

Div 

Div 

Div 

Div 

Div 

Div 

Div 

Div 

Div 

Div 

Div 

Div 


Sion 1 — Ballistic Research 
Sion 2 — Effects of Impact and Explosion 
Sion 
sion 
sion 
sion 
sion 
sion 
sion 


3 — Rocket Ordnance 

4 — Ordnance .\ccessories 

5 — New Missiles 

6 — Stib-Sui face Warfare 

7 — l ire Control 

8 — Explosives 

9 — Chemistry 

sion 10 —Absorbents and .\erosols 
sion 11 — Chemical Engineering 
sion 12— Transportation 
sion 13 — Electrical Communication 
sion 14 —Radar 
sion 15 — Radio Coordination 
sion IG —Optics and Camouflage 
sion 17 — Physics 
sion 18 — ^Var Metallurgy 
ision 19 — Miscellaneous 
Applied Mathematics Panel 
.Applied Psychology Panel 
Committee on Propagation 

Tropical Deterioration .Administrative Committee 


iv 



labrary «f CTiiif^rcss 



201 .^ 


4909.S0 






























NDRC FOREWORD 


DECLASSIFIED 
By authority Secretary of 


AS EVENTS of the years preceding 1940 revealed more 
and more clearly the seriousness of the world 
situation, many scientists in this country came to 
realize the need of organizing scientific research for 
service in a national emergency. Recommendations 
which they made to the White House were given care¬ 
ful and sympathetic attention, and as a result the 
National Defense Research Committee [NDRC] was 
formed by Executive Order of the President in the 
summer of 1940. The members of NDRC, appointed 
by the President, were instructed to supplement the 
work of the Army and the Navy in the development 
of the instrumentalities of war. A year later, upon the 
establishment of the Office of Scientific Research and 
Development [OSRD], NDRC became one of its 
units. 

d he Summary Technical Report of NDRC is a 
conscientious effort on the part of NDRC to sum¬ 
marize and evaluate its work and to present it in a 
useful and permanent form. It comprises some sev¬ 
enty volumes broken into groups corresponding to 
the NDRC Divisions, Panels, and Committees. 

The Summary Technical Report of each Division, 
Panel, or Committee is an integral survey of the work 
of that group. The first volume of each gioup’s report 
contains a summary of the report, stating the prob¬ 
lems presented and the philosophy of attacking them 
and summarizing the results of the research, develop¬ 
ment, and training activities undertaken. Some vol¬ 
umes may be “state of the art” treatises covering 
subjects to which various research groups have con¬ 
tributed information. Others may contain descrip¬ 
tions of devices developed in the laboratories. A 
master index of all these divisional, panel, and com¬ 
mittee reports which together constitute the Sum¬ 
mary Teclinical Report of NDRC is contained in a 
separate volume, which also includes the index of a 
microfilm record of pertinent technical laboratory 
reports and reference material. 

Some of the NDRC-sponsored researches which 
had been declassified by the end of 1945 were of suffi¬ 
cient popular interest that it was found desirable to 
report them in the form of monographs, such as the 
series on radar by Division 14 and the monograph on 
sampling inspection by the Applied Mathematics 
Panel. Since the material treated in them is not 


duplicated in the Sumn^f^T^chnjig^QReport of 
NDRC, the monographs are an important part of the 
story of these aspea.so£ NDRg^es(^rj^t,gu 3 t iggp 

In contrast to tlreimormati on on rad ar, which is of 
widespread interesj^|g|^ij^ 5 ^ to 

the public, the research on suljsurface warfare is 
largely classified and is of general interest to a more 
restricted group. As a consecpience, the report of 
Division 6 is found almost entirely in its Summary 
Eechnical Report, which runs to over twenty vol¬ 
umes. The extent of the work of a Division cannot 
therefore be judged solely by the number of volumes 
devoted to it in the Summary Technical Report of 
NDRC: account must be taken of the monographs 
and available reports published elsewhere. 

Any great cooperative endeavor must stand or fall 
with the will and integrity of the men engaged in it. 
This fact held true for NDRC from its inception, and 
for Division 6 under the leadership of Dr. John T. 
Tate. I’o Dr. l ate and the men who worked with 
him—some as members of Division 6, some as repre¬ 
sentatives of the Division’s contractors—belongs the 
sincere gratitude of the Nation for a dilficult and 
often dangerous job well done. 1 heir ellorts contrib¬ 
uted significantly to the outcome of our naval opera¬ 
tions during the war and richly deserved the warm 
response they received from the Navy. In addition, 
their contributions to the knowledge of the ocean 
and to the art of oceanographic research will as¬ 
suredly speed peacetime investigations in this field 
and bring rich benefits to all mankind. 

The Summary Technical Report of Division 6, pre¬ 
pared under the direction of the Division Chief and 
authorized by him for publication, not only presents 
the methods and residts of widely varied research and 
development programs but is essentially a record of 
the unstinted loyal cooperation of able men linked in 
a common effort to contribute to the defense of their 
Nation. To them all we extend our deep appreciation. 


Vannevar Bush, Director 
Office of Scientific Research and Development 


J. B. CoNANT, Chairman 
National Defense Research Committee 







FOREWORD 


T his volume deals with the physical properties of 
the medium in which subsurface warfare is waged. 
It is much more than an account of the oceanographic 
research sponsored by the Division. It is a text in 
which the science of physical oceanography is pre¬ 
sented with special reference to the significant appli¬ 
cations of that science to naval operations. Because of 
this, it should prove to be of wide interest to all Navy- 
personnel. 

The Division is deeply indebted to Mr. C. O’D. 
Iselin, Director of the \V"oods Hole Oceanographic 
Institution, not only for his willingness to undertake 
the task of collecting and editing the material pre¬ 
sented in this report, but also for the fruitful partici¬ 
pation of his institution in the research and develop¬ 
ment program of the Division. 


Special acknowledgment should also be made to 
the contributions from the Scripps Institution of 
Oceanography of the University of California. The 
staff of that institution provided not only broad 
knowledge of the problems involved but also special¬ 
ized knowledge of conditions in the Pacific Areas. 

From the beginning this research program has re¬ 
ceived the most cordial support of the Navy. The 
Bureau of Ships, Hydrographic Office, and forces 
afloat united to assist in every way both the progress 
of the work and the application of the results to 
operations. 

John T. Tate 
Chief, Division 6 



>ii 


RESIRICT 





PREFACE 


P HYSICAL oceanography is one of the most back¬ 
ward of the geophysical sciences. In general it can 
be said that until cjuite recent years the physical as¬ 
pects of the ocean were studied mainly as an adjunct 
to marine biology. The practical applications seemed 
to lie in fisheries problems, and the thinking of some 
oceanographers has been influenced accordingly. 
Others have been mainly concerned with the geo¬ 
graphic approach. More recently a number of me¬ 
teorologists have become interested in the circulation 
problem in the sea because of the several analogies 
with atmospheric problems. 

Perhaps for these reasons one finds, on the whole, 
rather little in the standard oceanographic text books 
that is helpful in understanding the transmission of 
sound in sea water. The chief aim of the present vol¬ 
ume has been to remedy this situation. It is hoped 
that it may be useful to those continuing with under¬ 


water acoustics to have available a simple summary 
of those aspects of physical oceanography which help 
to explain the behavior of sound in sea water. Since 
up to the present time rather little oceanographic re¬ 
search has been carried on in conjunction with 
acoustical studies, to those trained in the laboratory 
sciences this volume may seem rather elementary and 
none too well documented. Nevertheless, at the pres¬ 
ent stage of physical oceanography, we doubt that it 
would be profitable to treat the subject more ex¬ 
haustively. The fact remains that our knowledge con¬ 
cerning the ocean is still rather superficial. 

It is hoped that in the future underwater acoustics 
and physical oceanography will develop in close asso¬ 
ciation. If the present volume does nothing more 
than to indicate how closely these two subjects are 
intertwined, it will have served a useful purpose. 

C. O’D. ISELIN 


RESTRICTS 











CONTENTS 


PART I 

PHYSICAL OCEANOGRAPHY AND 
SUBSURFACE WARFARE 

CHAPTER . page 

1 Introduction.3 

2 The Development of Methods for Predicting Sonar and 

Diving Conditions.9 

3 Transmission of Sound in Sea Water.17 

4 Submarine Diving Problems.22 

PART 11 

TEMPERATURE AND SALINITY OF 
OCEAN WATERS 

5 The Basic Vertical d hermal Structure of the Oceans ... 27 

6 Relationship of Salinity and Temperature.51 

PART HI 

GEOGRAPHICAL AND LOCAL VARIABILITY 

1 Ocean Currents.59 

8 Local Variability.72 

9 Coastal Waters.77 

Clossary.97 

Bibliography.99 

Contract Numbers. 102 

Service Project Numbers.103 

Index.105 


RESTOIC'IED, 

^ 


XI 













PART I 

PHYSICAL OCEANOGRAPHY AND SUBSURFACE WARFARE 




* 



1 


4 


4 


4 



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It. 





^i> 


I 


4 


C 






Chapter 1 

INTRODUCTION 


AS inK inslrunicntaiioii of warfare at sea has cle- 
CT \eloped, and as these instruments are used by 
skillful operators from faster ships or at more nearly 
their extreme range, the linal limitation is frecjuently 
imposed by the {jhysical characteristics of the me¬ 
dium which vary both seasonally and geographically, 
'rhus in a \ cry real sense there is an occanograj^hy of 
naval warfare. 

As far as sid)surfacc warfare is concerned, naval 
occanograj)hy has already developed far enough to 
jjlay a ])art in both operational and materiel activ¬ 
ities. Knowledge of the sound conditions often makes 
it jDOSsible to use sonar gear more elfcctively and is 
sometimes a factor in deciding the proper tactical 
deployment of vessels. Knowledge concerning the 
sulrstirface distribution of temperature and salinity 
improves the diving operations of submarines and to 
a considerable degree affects their choice of offensive 
and evasive manetners. Oceanographic limitations 
have caused considerable modification of the stand¬ 
ard procedtires for operating sonar gear and have in 
some cases influenced its design. They have also re- 
cpiired the development and manufacture of instru¬ 
ments to aid in predictions. 

As time goes on, it becomes increasingly clear that 
just as climate and terrain affect the strategy of 
armies, so seasonal and geographical characteristics 
of the oceans must enter into the planning of the 
Navy. Whnds, waves, and currents influence subsur¬ 
face warfare, as well as aerial and amphibious opera¬ 
tions, in ways that are new and different but no less 
real than the limitations they placed on the old 
Navies, d'here are jiarticular times and places best 
suited to the ojK-ration of the diverse and specialized 
modern fighting ships, and skillful planning includes 
oceanograjjhy in the list of operational factors to be 
considered. 

It is the primary purpose of this report to discuss 
the general principles of oceanography, particidarly 
in relation to stdisurface warfare. .Special emphasis is 
placed on the parts of the sid)ject that are most inter¬ 
esting from the practical standpoint, namely the tem¬ 
perature and salinity of the upper few hundred feet 
of water and their seasonal and geographical varia¬ 
tions, winds and wa\'es and their effects on sound con¬ 


ditions, and bottom sediments in relation to shallow- 
water echo ranging. However, the disctission is not 
limited entirely to matters of immediate practical 
importance since a more general knowledge of physi¬ 
cal oceanography is essential for proper intei jneta- 
tion of sonar charts and bathythermograph records. 
I'he theoretical aspects of the subject are necessarily 
dealt with in a brief and nontechnical fashion here. 
For more complete treatment there are several stand¬ 
ard textbooks of oceanography available. 

I’hysical oceanography has been somewhat slower 
to develop than the other branches of the geophys¬ 
ical sciences, in part because of the expense and diffi¬ 
culties of carrying out research on shipboard and in 
part because until recently it was not generally real¬ 
ized how many practical ad\antages were to be 
gained through increased knowledge of the physical 
characteristics of the ocean. It was the biological as¬ 
pects of oceanography which were at first empha¬ 
sized, rather than the physical. 

I’he Germans seem to have been early aware of the 
potential importance of physical oceanography to 
modern naval operations, but it is not known at this 
time how far they developed the sidjject in relation 
to underwater sound transmission. It is known that 
during the period between the two world wars, the 
Deutsche .Seewarte at Hamburg and the Institut fiir 
IMeereskunde at Berlin were strongly supported by 
the go\ernment, and the German Navy had an ocean¬ 
ographic laboratory at ANhlhclmshafen. Able staffs 
were assembled and German naval vessels were used 
in extended field surv eys. One result of this active in¬ 
terest in oceanography on the })art of the German 
Navy, namely an atlas of the changes in density of the 
water, both horizontally and vertically, was found in 
1911 on a caj)tured German submarine. This was 
designed primarily to guide the diving officer, but 
nothing so definite can be cited in connection with 
sound transmisson. The Jajianese government has 
also taken an active interest in hydrological surveys. 

Although in our country the Hydrographic Office 
had pioneered in the early development of the sub¬ 
ject, when currents and weather at sea were more 
important to navigation than they are today, of 
recent years the further exploration of the ocean has 




4 


INTRODUCTION 


been left largely to private institutions. Before the 
present tvar only a very small beginning had been 
made by our Navy in the study of the physical factors 
influencing underwater sound transmission or the 
operation of submerged submarines. The relatively 
few physical oceanographers in this country were 
only ^ aguely aware of how their studies might be of 
interest to the Navy. 

There have been four active centers of research in 
physical oceanography in this country during the 
war years: namely, the Oceanographic Unit of the 
Hydrographic Office, the Scripps Institution of 
Oceanography [SIO], at La Jolla, the Oceanograjjhic 
Section of the University of California Division of 
War Research [UCDWR], at San Diego, and the 
Woods Hole Oceanographic Institution [W^HOI]. As 
far as the oceanography of subsurface warfare is con¬ 
cerned, the two latter laboratories have carried out 
more of the research, but the interests of these four 
groups have been so close that it would be difficult in 
this report to place credit where it is due. In fact, 
even the personnel has been freely exchanged. In 
general, of course, at Woods Hole the studies have 
dealt particularly with the Atlantic Ocean, while for 
the most part at San Diego and La Jolla data from 
the Pacific have been analyzed. 

It is perhaps worth noting in passing that ocean¬ 
ography is of importance to the Navy in a number 
of ways cpiite different from those discussed here 
and, therefore, at some of these laboratories active 
research has been in progress which is more or less 
unrelated to submarine warfare. For example, on the 
biological side studies ha\e been carried out of the 
fouling of underwater surfaces. In mine warfare the 
currents and the character of the bottom sediments 
sometimes must be taken into account. In amphibi¬ 
ous operations results of research on waves are im¬ 
portant. To some extent these few examples have 
been cited to explain why the few qualified oceanog¬ 
raphers available in this country at the start of the 
war have had to take iqj many and diverse studies. 
I'hey have been ably assisted by scientists and tech¬ 
nicians from related fields, but there has been too 
little time as yet to develop any one of the practical 
naval aspects of the subject as fully as is perhaps 
warranted. 

It is therefore worth considering in this volume 
not only what has been done but also what problems 
remain unsolved. VVhth both these aims in mind, the 
report begins with an account of the investigations 


that were undertaken to determine the effects of 
oceanography on subsurface warfare, the instru¬ 
ments that were developed, the methods of using the 
instruments for prediction purposes, and the ways in 
which these methods might be improved. It con¬ 
tinues with a chapter on the transmission of sound 
in sea water and another on submarine di\ ing prob¬ 
lems. Since other volumes of the series deal specific¬ 
ally with these subjects, both are treated summarily 
here, giving only such facts as arc needed for the prac¬ 
tical interpretation of the oceanographic material 
that follows. I'he latter is di\ ided into two parts. One 
deals primarily with the temperature and salinity 
structure of the ocean and its daily and seasonal 
changes due to heating, cooling, evaporation, and 
the various other physical processes that take place at 
the surface of the sea. The other is concerned with 
the effect on sound conditions of geographical and 
local variability of the oceans. Here are discussed 
ocean currents and eddies, coastal waters and bottom 
sediments, and special phenomena associated with 
winds and tides. 

1 ' PREWAR INVESTIGATIONS OF THE 
ROLE OF REFRACTION IN UNDERWATER 
SOUND TRANSMISSION 

As far as is known, in this country the role of re¬ 
fraction in echo ranging was first seriously con¬ 
sidered in 1937 when some transmission measure¬ 
ments were made off Guantanamo, Cuba, accompa¬ 
nied by measurements of the ^ertical temperature 
structure of the water. The latter was determined by 
means of closely spaced mercurial thermometers. It 
was shown that the decrease in the speed of sound 
with depth, due to the decrease of temperature with 
depth near projector level, could sometimes cause 
sufficient downw^ard refraction so that the beam 
passed beneath a shallow target, except at relatively 
short ranges. Very shortly afterwards the same idea 
was advanced independently at the ^Vest Coast 
Sound School [WCSS], San Diego. It is important to 
note that both off Guantanamo and San Diego the 
echo-ranging operations were carried out in deep 
water in areas where the effects of downward refrac¬ 
tion are particularly striking. A large percentage of 
the earlier tests of sonar gear had been conducted in 
places where refraction effects were not so frequent. 

In the course of the transmission measurements off 
Guantanamo, and during a second cruise off New 


RESTRICTED^ 









DEVELOPMENT OF THE SURFACE VESSEL BATHYTHERMOGRAPH 


5 


London the following summer, it became obvious 
that an instrument which would record temperature 
continnonsly against deptli as it is lowered into the 
sea would have a great ad\antage in refraction studies 
over the conventional deep sea reversing thermome¬ 
ters of oceanography. A crude model of such an in¬ 
strument was at that time being tested at the Woods 
Hole Oceanographic Institution. This instrument 
had been named the bathythermograph [BT] and 
was in fact actually used on a few occasions during 
the second series of transmission measurements. 
Modifications residted in a somewhat improved B'L 
which gave fair residts when used from a \essel mo\- 
ing at 6 or 7 knots and which was used successfully for 
oceanographic studies.’'- ^ 

12 DEVELOPMENT OF THE SURFACE 
VESSEL BATHYTHERMOGRAPH 

Beginning in October 1940, the performance of the 
BT was further modified by WHOI working under 
contract with the National Defense Research Com¬ 
mittee with the idea of using it as a naval instrument. 
It was then realized that for use by the surface vessels 
of the Navy such an instrument had to be workable 
at speeds of at least 12 to 15 knots, and this imposed 
\ery definite limitations on the design. It will be 
worth while here to discuss briefly these limitations. 

First of all it was essential to have a light, rugged 
instrument that could utilize the sounding-lead prin¬ 
ciple of falling freely through the water in order to 
gain sufficient depth before the drag of the wire by 
which it is recovered becomes limiting. The alter¬ 
native would be to give the instrument diving fins to 
aid in its descent, which would recpiire in turn a 
heavier cable to retrieve it and a more powerful 
winch, thus increasing both the expense and the 
difficulties of handling the gear. 

In order to exploit the sounding-lead principle 
the instrument must have a very rapid thermal re¬ 
sponse. Otherwise it could not record the vertical 
temperature changes accurately. This is desirable 
from another standpoint as well. Any instrument 
which is either lowered or raised at less than the 
maximum possible speed begins to be influenced by 
horizontal temperature variations. This could be 
o\'ercome in part, it is true, by causing the instrument 

a 'Hie iiistiuniciil used in this work was the otitgrowth of a 
preliminary model called an “oceanograph,” which was built by 
Rossby anci Lange at WHOI in 1934. 

• ' ^ 

rest: 


to record only as it is lowered or raised. However, the 
only way to be certain that the thermal element is 
functioning properly is to record most of both the u]) 
and the down temperature changes. Agreement be¬ 
tween the two traces is proof that vertical and not 
horizontal gradients have been measured. 

Another major reejuirement is that the tempera¬ 
ture record must be relatively free from the effects of 
vibration so that Aery slight temperature changes 
Avith depth can be obseiAed, especially in the upper 
50 feet of the Avater column. This necessity practically 
precluded the use of diving planes. Also, it again 
raised the (juestion of Avdiether or not the instrument 
should be constructed to record only the doAvn trace, 
since Aibration is most likely to occur during the 
raising. It Avas decided that such vibration as oc¬ 
curred Avas not serious enough to offset the advan¬ 
tage of having both traces recorded on the slide. Since 
that time, hoAveAer, there has been an increasing ten¬ 
dency to take readings at high speeds, thus increas¬ 
ing the vibration and sometimes making it difficult 
to interpret the records. Vibration is therefore the 
greatest Aveakness of the surface vessel BT discussed 
here, and modifications may be required. 

The instrument must be used repeatedly from a 
relatively fast ship, making it almost impossible to 
have it record on deck. Insulated electric cables Avould 
never stand use on a high-speed Avinch. Finally, be¬ 
cause at best there is a good chance that the Avire may 
become fouled in the screAVs resulting in the loss of 
the instrument, it should be inexpensive and easy to 
manufacture. Thus a mechanical type of thermal 
clement is almost a necessity. 

These considerations and the tests carried out dur¬ 
ing the autumn of 1940 in the Avestern North Atlantic 
led to a simple, rugged design Avhich has not been 
essentially altered since. A Bourdon-type thermal 
element, Avhich is compensated by a bimetallic strip 
to take care of the difference in temperature betAveen 
the water inside the instrument and that through 
Avhich it is passing, actuates a pen arm that records 
on a smoked glass slide mounted rigidly on the pres¬ 
sure element (Figures 1-3). The pressure element is 
of the belloAvs type, having a stiff internal spring and 
an accurately machined internal guide rod. The 
effect of increasing external pressure is to shorten the 
length of the belloAvs. 

'I he speed of response of the thermal element is 
such that about 80 per cent of the full temperature 
range can be achicAcd in less than 1 second. AVTile 










6 


INTRODUCTION 



TEMPERATURE ELEMENT PRESSURE ELEMENT 




records vertical temperature gradients accurately 
enough for practical purposes when combined with a 
good pressure element. A rough temperature calibra¬ 
tion can be maintained, pro\ ided the surface tem- 
jierature is observed with a reliable mercurial ther¬ 
mometer. 

Early in the development of the BT a basic deci¬ 
sion had to be made. Should it record temperature 
against depth or, on the assumption that salinity is 
constant with dejjth, should it record the speed of 
sound? To have the instrument read directly in terms 
of the velocity of sound was olD^■iously the direct ap¬ 
proach to the refraction problem, yet in the end other 
considerations outweighed this one. Approximately 
70 to 80 per cent of the time there exists near the 
surface a virtually isothermal layer which is stirred 
by convection and by the wind. However, this is not 
an isovelocity layer, because of the effect of pressure 
on the speed of sound. Thus the proper performance 
of the thermal element can be much better judged in 


the absolute accuracy of such a thermal system is not 
notably great unless it is frequently calil^rated, it 


Figure 2. Photograph of installation. 


RESTRICTED 












































































































































































THE BATHYTHERMOGRAPH FOR StIBMARINES 


7 


terms of temperature than in terms of Aelocity. Fur¬ 
thermore, the oceanographic aspects of sound trans¬ 
mission are much more easily understood in terms of 
temperature. As a residt of this decision, the empha¬ 
sis in the present report is on temperature rather than 
on changes in the speed of sound. At the same time it 
was decided to use the Fahrenheit temperature scale 
in accordance with Navy practice. 

13 THE BATHYTHERMOGRAPH FOR 
SUBMARINES 

During the course of the development of the sur¬ 
face vessel BT it became evident that a corresponding 
instrument for submarines woidd have several prac¬ 
tical applications. During the summer of 1941, in 
response to a request by the Commander of Subma¬ 
rines, Atlantic Fleet, a number of temperature-depth 
recorders were built at Woods Hole and tested in sub¬ 
marines both off New London and off Key WAst. At 
first the BT for sid^marines was thought of primarily 
as an acoustic instrument. 1 hat is, it wotdd be used 



Figure 3. Photograph of a l)ath\ thermogram. 


to predict maximum range of the sonar equipment 
on the submarine, as well as the performance of her 
adversary’s gear. However, early in the tests off Key 
West it became evident that vertical temperature 
changes could be interpreted in terms of density with 
sufficient accuracy to be of considerable assistance to 
the diving officer in maintaining proper trim. Thus a 
second use arose for the instrument, which has be¬ 
come at least as important as its original purpose. 



COUNTER 

WEIGHT 


TUBING IN BLISTER 
OUTSIDE CONNING TOWER 


HOLDER 


XYLENE FILLED 


TEMPERATURE 
BOURDON TUBE 


STYLUS 


COUNTER 

WEIGHT 


Figure 4. Diagram of submarine bathythermograph. 


^ESIRICTEI^ 

































































































































INTRODUCTION 


« 


riic best location lor the thermal element on the 
hull ol the submarine is still a matter to be settled by 
deliniti\'e tests. Locatint^ it high on the conning tower 
has in the j^ast been an acceptable compromise be- 
tweeti a still higher position which would be preler- 
able lor acoustical purposes and a lower position 
which would laxor dicing. It is possible that two in¬ 
stallations Avill be used later. Except lor this ejues- 
tion, the initial design jjroblem ol the BT lor sul)- 
marines evas not a difiicult one. It was a matter ol 
building a sufficiently rugged and simple instrument 
which would lit into the acailable space. .Again a 
Bourdon-tyjie thermal element was used with suit- 
able compensation lor temperature changes inside 


the hull ol the sitbmarine. .Another Bourdon tulie 
was used as a pressure clement by connecting it with 
the pressure line leading to a depth gauge. The gen¬ 
eral features ol the installation are shown diagratn- 
maticalh in Figure 2. 

From the standpoint ol sulisurlace warfare, the 
density ol sea evater is relaticely somewhat more sen- 
sit i\c to changes in salinity than is the speed ol sound 
(see Section .S.2.3). I'hcrelore recenth there has been 
under decelopment a so-called salinity-comjiensated 
BF for sidmiarines.'’ Ibis measures temperature, 
salinity, and pressure, and (omputes and records 
lioth ballast change and the speed ol sound against 
depth. 





Chapter 2 

THE DEVELOPMENT OF METHODS FOR PREDICTING 
SONAR AND DIVING CONDITIONS 


T he early investigations of sound conditions that 
led to the development of the bathythermograph 
[BT] have been continued. It (juickly became ap¬ 
parent that the whole problem of sound transmission 
in sea water was highly complex and that echo and 
listening ranges might be limited by any one of half 
a dozen or more factors. A great deal of fundamental 
research was therefore necessary before it was pos¬ 
sible to devise a simple and accurate method of trans¬ 
lating the basic temperature-dejJth curve of the BT 
into a practical prediction of sonar conditions. 

Wartime exigencies rccjuired that as (|uickly as 
possil)le the results of investigations l)e turned into 
practical information for the men on the ships. Pre¬ 
diction methods ha\e therefore been revised fre- 
(juently as new research suggested the desirability of 
modifying existing methods. Proper caution was ex¬ 
ercised in accepting such modifications, and so far 
onlv one extensive revision of official procedure has 
been retpiired; nevertheless, the rapid developments 
in this field have resulted in .some confusion, which, 
however unfortunate, seemed to be justified by the 
needs of the moment. 

.Aside from manuals on the operation and main¬ 
tenance of the Bl', official literature on the subject 
can be classified into two main groups: 

1. Prediction manuals. These are designed so that 
the olxserver on board shij) can make practical use of 
any particular bathythermogram by determining 
within certain probable limits the range that can be 
obtained on a submarine and can therefore operate 
the shij) and sonar eciuijjment according to the most 
efficient tactical usage for that jiarticular range. The 
sulimariner obtains information similarly useful to 
him on the best depth for evasion, probable ping and 
listening ranges, and the most efficient diving pro¬ 
cedure. 

2. Charts. These include sonar charts showing 
average echo ranging conditions, average diving con¬ 
ditions (in the Submarine Supplements), and bottom 
.sediment charts for shallow-water sonar work. Charts 
of average conditions arc of less tactical value than a 
bathythermogram obtained at the time and place 
needed, becau.se conditions are generally too variable 


to permit accurate enough predictions on the basis 
of averages. On the other hand, they provide a per¬ 
spective unobtainable from a small number of bathy- 
thermograms and hence are useful not only to the 
observer for determining how often BT lowerings 
should be made, but also for more important stra¬ 
tegic purposes. 

It is beyond the scope of the present work to dis¬ 
cuss the Navy BT literature and its scientific origins 
in detail. However, a more limited discussion is war¬ 
ranted becatise frequent reference will be made 
throughout the volume to the official literature in 
connection with the oceanographic part of its back¬ 
ground. The history and the purpose of the predic¬ 
tion methods will therefore be described briefly. 
Wdiere obvious improvements can be made, either by 
collection of more observations or by better irse of 
existing data, these will be mentioned. 

2' PREDICTION MANUALS 

In February 1941 a report^ was published sum¬ 
marizing what was then known about the refraction 
of sound in .sea water and the oceanographic factors 
chiefly responsible, llie present report is in part an 
expansion of the oceanographic section of this earlier 
study which was based largely on the experience 
gained at the time of the early transmission measure¬ 
ments previously mentioned. 

Since the idea that sound is refracted in sea water 
in accordance with the principles of geometrical op¬ 
tics was not generally accejJted, much of the discus¬ 
sion in this early report was in support of a simple 
refraction theory. The report was especially con¬ 
cerned with demonstrating that as a first approxima¬ 
tion the vertical temperature distribution alone pro- 
\ ided a good means of calculating the refraction pat¬ 
tern. Such calculations were later simplified by the 
introduction of a refraction slide rule,2 and recently 
there has been published a collection of a large num¬ 
ber of such calculations for the common vertical 
temjicrature distributions.-'* Another contribution 
was the development of the sonic ray plotter.^ 

More recent sttidies of the transmission of standard 


^RF.S I RICTEI) ^ 


9 




10 


METHODS FOR PREDICTING SONAR AND DIVING CONDITIONS 


sonar gear have shown that the clearly defined acous¬ 
tic shadow zones which are predicted on the basis of 
geometrical optics are by no means always observed. 
Only in the case of sharp downward refraction does a 
marked acoustic shadow zone appear. The Aveak- 
nesses of the simple refraction theory are not yet fully 
understood and at the time of this Avriting sound 
field measurements are still being actively secured. 
On the other hand, the reduction in sound intensity 
beloAV a sharp thermocline, the so-called layer effect 
in echo ranging, Avhich is predicted by the simple 
theory, does in fact exist. Thus no matter Avhat the 
final result of the transmission studies may be, it is 
clear that under contemporary operating conditions 
vertical temperature gradients, especially Avhen they 
are marked, are often the limiting factor in subma¬ 
rine detection. It is also clear that as the sensitivity of 
sonar equipment is increased and as submarines be¬ 
come more skillful in taking advantage of the oceano¬ 
graphic factors in their faAor, success in subsurface 
Avarfare Avill depend more and more on Avhich side 
has the better understanding of the medium. 

According to current theories of antisubmarine 
and prosubmarine Avarfare, there are certain advan¬ 
tages to be gained by knoAving the changes of maxi¬ 
mum sonar range Avith depth. In addition, it is gen¬ 
erally agreed that our submarines can change depth 
more cpiickly and more silently if the diving officer 
understands the changes in density in the superficial 
layers of the ocean. 

Although the scientific theory of sound transmis¬ 
sion in sea Avater remained undeA eloped in many of 
its details, it gradually became possible by synthesis 
of the available scientific information Avith data on 
obsei'Acd maximum echo ranges to Avork out simple 
schemes for range predictions that are accurate 
enough for practical Navy purposes. Such schemes 
Avere pid^lished in the official prediction manuals.'’ '’ 

A number of problems arose in Avriting the man¬ 
uals. In the first place, it had l)ecome clear that the 
refraction slide rule method of calculating acoustical 
ray diagrams A\ as too ponderous and time-consuming 
to be of much practical value at sea. It Avas not 
enough to perform the relatiAely simple operation of 
determining the limits of the so-called shadoAv zone, 
because except in the case of strong doAViiAvard re¬ 
fraction, the intensity of sound near the outer limit of 
the direct sotind field Avas generally too Ioav to return 
a detectable echo. It Avas necessary to go further and 
to calculate the distribution of intensity Avithin the 


beam. I'his Avas a long task generally requiring 2 
hours’ Avork or more by an experienced calculator, 
and it Avas hardly to be recommended as a standard 
Navy procedure. 1 he alternatiAe Avas a method of 
classifying B'F slides according to the most significant 
features of their temperature structure, such as the 
depth of the mixed surface layer (knoAvn as layer 
depth in XaAy literature) and the strength of nega¬ 
tive gradients near the surface and beloAV the mixed 
layer. Then the probable echo range could be pre¬ 
dicted according to simple rules for each type of tem¬ 
perature pattern. 

Any such method has both adAantages and disad- 
Aantages. The vertical temperature structure of 
ocean Avaters is at times complex, and no simple 
scheme for classifying it Avill be accurate in all cir¬ 
cumstances. The methods in the manuals reduce 
such errors to a relatively Ioav percentage but do not 
entirely eliminate them. In fact more careful scien¬ 
tific methods have thus far failed to shoAv complete 
correlation betAveen bathythermograms and observed 
ranges. The discrepancies are presumably due to a 
combination of seA eral factors such as absorption and 
scattering of sound, and small and frequently chang¬ 
ing variations in the refraction pattern that cannot 
be determined easily. These factors haA e not and per¬ 
haps never can be fidly eA aluated. 

But even if the oceanographic variables Avere un¬ 
derstood completely, it Avould be a mistake to sup¬ 
pose that maximum echo ranges coidd be forecast 
Avith great precision under practical circumstances. 
There ahvays remain unforeseeable factors, such as 
target asjiect and speed, that may introduce varia¬ 
tions in maximum range of as much as plus or minus 
one-third of the prediction. With such large unavoid¬ 
able errors in the predictions it Avould be fruitless to 
complicate the system by small and relatively insig¬ 
nificant refinements in refraction classification. 

MoreoAer, a manual must take into account the 
fact that the predictable part of sonar performance is 
not ahvays determined by Avater conditions. Indeed 
it is difficult to form an estimate of hoAv much of the 
time the oceanographic factors are really limiting. 
If the antisubmarine vessel is a destroyer or destrover 
escort and is not operating at too high a speed, if 
standard heavyAveight sonar gear is being used, if the 
operator is skillful, and there is no electrical or me¬ 
chanical trouble, then in deep Avater either the 
Aveather or the temperature distribution may become 
limiting. Thus oceanographic factors may be limit- 









PREDICTION MANUALS 


II 


ing only a small percentage of the time or most of the 
time depending on the ship, its equipment, sonar 
personnel, and type of duty. 

I'he non-oceanographic factors can be evaluated 
for any particular ship and any set of operating con¬ 
ditions by means of a few experiments. The current 
manual for the surface vessel BT (see reference 5) was 
intended primarily for ships of the destroyer escort 
class or larger, using standard heavyweight sonar 
ecjuipment. The manual therefore evaluates prob¬ 
lems of echo ranging as they apply to this particular 
type of vessel, with proper allowances for self noise 
and the effect of heavy weather in reducing ranges. It 
assumes that the gear is operating properly, but to 
make sure this is so, ways are suggested for testing its 
performance. Thus it is not merely a manual of re¬ 
fraction conditions but is a realistic approach to the 
whole problem of echo ranging. 

The prediction manual for submarines (see refer¬ 
ence 6) is similar in basic plan to the manual for sur¬ 
face ships. Prediction methods are not completely 
uniform, but the differences are a result of the special 
c[ualities and needs of submarines. The submarine 
has a much ■wider \ ariet)' of uses for the BT than does 
the surface vessel. Perhaps the most important use is 
for predicting ballast adjustments so that diving can 
be accomplished as quietly as possible and with a 
minimum of lost time. From the standpoint of acous¬ 
tics it is used to determine various things such as the 
l)est depth for evasion, ping ranges and listening 
ranges on surface ships, and to a lesser extent maxi¬ 
mum echo ranges at periscope depth and at various 
other depths for use in evasion. 

Each of these acoustical predictions is a separate 
problem in which refraction plays a role but is more 
or less modified by other factors of a non-oceano¬ 
graphic nature. In some respects, the submarine at 
periscope depth is in a more favorable position for 
echo ranging than a surface vessel. Self noise can be 
reduced to a low level, and its deep projector and its 
stability in heavy weather reduce cjuenching and 
pitching effects to a minimum. However, both the 
greatest range at which a submariner can obtain an 
echo from a surface vessel, and the greatest range at 
which he can hear the pings of a surface vessel trying 
to echo-range off him are affected by the speed of the 
surface vessel, though in different ways. Prediction of 
maximum listening ranges must include allowances 
for submarine and target ship speed as well as wind 
force. 


The submariner’s knowledge of refraction condi¬ 
tions and his practical utilization of them have been 
limited by the fact that the temperature element was 
mounted on the conning tower, so that at periscope 
depth it was impossible to determine the temperature 
structure in the upper 20 feet of water. Therefore, 
submariners have not had the knowledge that would 
permit them to make full use of the short periscope 
depth ranges that occur when there is a pronounced 
temperature decrease in the upper few feet. This, 
combined with the desirability of great depth for in¬ 
creased evasion time, means that in most cases a sub¬ 
marine will go deep when being attacked. However, 
new installations providing a temperature element 
on the periscope shears will allow more accurate de¬ 
termination of refraction conditions and enlarge the 
choice of evasive tactics. 

Prediction methods presumably will be improved 
in many small ways in further editions of the manu¬ 
als. In many cases the available information has not 
been so complete as might be desired. In time this 
situation will be corrected and the accuracy of pre¬ 
diction methods will be increased. There is great 
need for elaboration of acoustical methods in shallow 
water. It may ver\' well prove impossible to predict 
maximum echo and listening ranges in shallow water 
with any degree of precision, for it is too complex a 
subject to resolve itself easily into a few basic rules. 
However, there are tactical considerations such as 
selection of proper receiver settings and signal length 
and manipulation of tilting beam that can lead to 
great impro\ements in sonar performance. Predic¬ 
tions for echo ranging in deep water should also be 
improved if possible, particidarly in respect to ranges 
on a target at known depth. This becomes important 
with the advent of accurate depth-determining gear, 
since good predictions of range at the depth of the 
target will modify and improve the tactics of reattack. 

Far more important than these minor re\ isions of 
existing methods are the changes and additions that 
must be made in order to keep abreast of current 
Navy developments. New gear requires careful study 
to determine how oceanographic factors affect its 
operation. ^Vithin the short space of World AVar II, 
there were se\eral major developments in sonar 
equipment, and one of these in particular, the tilt¬ 
ing beam, has created new functions for the BT 
which are actually more important than those for 
which it was originally intended. Submarine listen¬ 
ing methods are expected to improve a great deal. 


RESTRICTED 









12 


METHODS FOR PREDICTING SONAR AND DIVING CONDITIONS 


This applies both to the equipment itself and to re¬ 
duction of self noise. In general, listening has not 
been studied nearly so thoroughly as echo ranging, 
and its full potentialities and the rules governing its 
use will not be de^'eloped for some time. 

New tactics may also have oceanographic implica¬ 
tions that are an important part of the statistical 
examination of their chances for success and may 
modify their use or determine whether or not they 
may be used in a particidar situation. All such devel¬ 
opments require changes in the existing manuals. 

22 SONAR CHARTS 

Early in the development of BT methods, only a 
few ships were equipped with the instruments and 
there was no possibility of making all necessary in¬ 
stallations within less than a year or so. The need was 
apparent for some sort of information to aid sonar 
personnel in understanding the effects of oceano¬ 
graphic factors on echo ranging and in knowing ap¬ 
proximately what kind of sound conditions might be 
expected in different parts of the oceans. The answer 
to this need was the sonar charts, which show average 
echo ranging conditions for all the oceans at differ¬ 
ent seasons of the year. 

Since their beginning the charts have gone through 
several revisions and have changed in both their 
form and in the purposes for which they are used. 
Whth the increase in the number of BT installations, 
ships are no longer dependent on charts for all of 
their information on sonar conditions. However, the 
charts ha\e been incorporated in the prediction pro¬ 
cedure and are used in conjunction with the BT to 
determine how often lowerings need to be made, to 
indicate layer depth whenever tactical conditions 
prevent a deep lowering, and for various other pur¬ 
poses. They are ecpially applicable to submarine op¬ 
erations, though the Submarine Supplements dis¬ 
cussed in Section 2.3 have in certain strategic areas 
replaced them with similar but more detailed infor¬ 
mation. And finally, since they are the best available 
information on what kind of sonar conditions can be 
expected at any given place and time, it is expected 
that the charts may serve a purpose in planning naval 
operations. 

The construction of these charts requires a nice 
balance between a purely statistical analysis of the 
available bathythermograms and general oceano¬ 
graphic knowledge. The data on the winds are on the 


whole adecjuate, but the distribution of bathyther¬ 
mograms and oceanographic stations for any one 
month is still most unsatisfactory. Even when the 
observations from three months are combined, there 
are very large areas without a single observation. 

If the observations were e\'enly scattered, both 
geographically and with time, a purely statistical 
treatment might not be seriously misleading. How¬ 
ever, in any given area, even as large as a 5-degree 
s([uare, it often happens that the available data are 
mostly from a single month and are not well distrib¬ 
uted. The result is that it is still advisable to use a 
certain amount of art in drawing the contours, even 
in the case of the Northern Hemisphere oceans for 
which roughly 200,000 bathythermograms and 
oceanographic stations are now a\ ailable. Except for 
the Southwest Pacific, there are less than 1,000 obser¬ 
vations in all three Southern Hemisphere oceans com¬ 
bined. Eigure 1 shows the positions where bathy¬ 
thermograms have thus far been obtained. Surpris¬ 
ingly, it has turned out that general oceanographic 
knowledge is on the whole adequate for this particu¬ 
lar purpose. The charts constructed before the bathy¬ 
thermograms were available in cjuantity differed only 
slightly from the newest editions as far as accuracy of 
oceanographic information is concerned. 

In successive revisions the sonar charts have gone 
through an evolution parallel to that of the predic¬ 
tion manuals. In the beginning they were based 
entirely on refraction, and the range contours on the 
charts simply represented average layer depth based 
on the known wind and current systems. In later 
editions other factors have been considered. 

Ehe charts have been expanded to contain the 
periscope depth range chart, and the assured range 
and layer depth chart. The first of these shows the 
a\erage percentage of time that the periscope depth 
range is reduced to less than 1,500 yards by unfavor¬ 
able tenq^erature gradients near the surface of the 
water or by strong winds of force 7 or more on the 
Beaufort scale (relationship of the Beaufort scale to 
other velocity scales is shown in Eigure 2).“ 


i‘ t he Reaufon scale was originally devised by Admiral Sir 
I rantis Beaufort in 1805 on the basis of how much canvas a man 
o' war of that time could carry under different winds. .\n at- 
lempi to place the scale on a more objective footing was made 
by Dr. G. C. Simpson, who.se revised scale was accepted in 1920 
h\ the International Meteorological C.ommittee. It is this re¬ 
vised scale which is presented graphicallv in Figure 2. It relates 
the Beaufort numbers to wind measurements made with an 
anemometer mounted in an open situation at a height of 33 feet 

CTEIT 









Figure 1. Chart showing-positions where Iiatln tlierinograms have liecn obtained. 


















































































































































































































f 


K- 








SUBMARINE SUPPLEMENTS 


13 


KNOTS 

O to 20 30 40 50 60 



Fk;ire 2. Beaufort scale of wind force compared with 
other velocity scales. 


"I'hc assured range and layer dej3ih chart shows the 
average range on a sidiinarine which is at the best 
depth lor avoiding detection (which may be peri¬ 
scope depth or just below layer depth), and it also 
shows the average layer depth. The range contours 
on this chart give the average range as indicated by 
temperature gradients when the wind is less than 
force 7. In regions where strong winds fretpiently re¬ 
duce the asstired range to less than 1,500 yards, the 
percentage of time this occurs is indicated. I'his type 
of chart depends primarily on large-scale oceano- 
grajjhic features which can on the whole be judged 
rather well from a knowledge ol the seasonal cycle. 
It will be improved considerably when more is 
known about the details of geographical xariations 
in the seasonal thermal cycle. 

I'hese two types of charts have been constructed 
for both winter and summer conditions in all the 
oceans. Whnter in the Northern Hemisphere is de- 

(10 meters) altove sea level. The height is important, since when 
the air is stable (i.e., with colder air underlying warmer air) the 
wind velocity may varv greatlv with altitude. .Such a situation 
is common when the sea surface is much colder than the air, as 
olf certain continental coasts in the summer time. I nder such 
circumstances, the reading of an anemometer mounted on a 
naval vessel's masthead is not necessarily a reliable indication of 
the elfecti\e wind force bv the Beaufort scale. 


fined as December through February and summer as 
June through Atigust. I'he same months are used in 
■Southern Hemisphere charts, but the seasons are of 
course reversed. For the North Pacific and North 
.Atlantic there are also three smaller charts that show 
graphically the monthly variation in the average 
periseope depth range, assured range, and layer 
dejjth. Fhus, in a general way the sonar charts show 
the whole seasonal temperature cycle for these areas. 

Future sonar charts will be improved with the 
gradual acejuisition of more Bd' records. There are 
very few areas in the oceans that have as yet a com- 
j)lete yearly cycle of BT observations, and there are 
\ery large areas where oljservations for only one 
month have been obtained. In such cases it is neces¬ 
sary until more records have been obtained to base 
the charts almost entirely on general oceanographic 
knowledge, wind observations, and a scattering of 
hydrographic stations. Collection of usal)Ie records 
will akso be accelerated by more accurate data accom¬ 
panying the bathythermograms. ^\'ithout the correct 
position, time, and date a BT slide is useless for pur- 
po.ses of analysis. Many of them have been discarded 
l)ecanse the positions were missing or obviously in¬ 
correct. Perhaps other errors not so obvious have es¬ 
caped attention and ha\e been responsible for some 
of the variability on the charts. 

It has been apparent in the preceding discussion 
that sonar ranges are dejjendent not only on various 
purely oceanographic factors but also on winds, 
weather, and climate in general, d'hese relationships 
will be cliscus,sed in some detail in the chapters that 
follow, although it will be seen that our knowledge 
about them is as yet liy no means complete. 

23 SUBMARINE SUPPLEMENTS 

Sid)marine Sujjplements to the Hydrographic 
Ofhee’s Stiiling Ditections for particular areas and 
seasons ha\e been issued to provide submariners with 
local infotmatictn about diving and sound condi¬ 
tions. .Although their general function is somewhat 
similar to that of the sonar charts, the scope is much 
Itroader. d'he Supplements contain charts of average 
diving and echo ranging conditions, but they also 
cli.scuss the oceanography of the region in respect tc:) 
effects of weather and seasonal climate, so that the 
usefulne.ss of the charts is increased by providing real 
understanding of local conditions. Previous experi¬ 
ence of submariners in the area is also made available 





























14 


METHODS FOR PREDICTING SONAR AND DIVING CONDITIONS 


by including excerpts of action reports that discuss 
the practical utilization of existing oceanographic 
conditions. 

The first Submarine Supplement, covering the Bay 
of Biscay area, was issued by the Hydrographic Office 
in June 1943, and it was followed shortly by a second 
one on late summer conditions in the Japanese Em¬ 
pire area. Their reception by submariners was favor¬ 
able enough to justify plans for expanding the pro¬ 
gram and publishing Supplements for all areas of 
strategic importance. 

I'he type of information in the different Supple¬ 
ments naturally has varied somewhat, depending on 
the oceanography of particular regions. In regard to 
sonar conditions, the most important feature has 
been data on the best depths for avoiding detection 
by echo ranging activities, and this has been sup¬ 
plemented with material on assured ranges, maxi¬ 
mum echo ranges at periscope depth, types of bottom 
in shallow water areas and their sonar significance, 
listening conditions, and local acoustic phenomena 
such as biological noise. Diving problems have been 
emphasized somewhat more than sonar conditions. 
Material on this subject has included ballast changes 
recpiired for di\ing and cruising in the area and 
salinitv corrections to BT data. 

j 

'I'he Submarine Supplements have not been in¬ 
tended for immediate tactical use. Figures on average 
conditions and proliable variations are poor substi¬ 
tutes for exploratory dives, although they are useful 
enough when an emergency prevents such dives from 
being made before beginning offensive or defensive 
action. On the other hand, an understanding in ad- 
\ance of conditions and a general knowledge of the 
appropriate type of operations to be used proves ad¬ 
vantageous from both the tactical and strategic stand¬ 
points. Strategy of larger scope, of planning a sub¬ 
marine campaign according to optimum balance be¬ 
tween desired objectives and necessary risk will also 
utilize oceanography of the type discussed in the 
Supplements, which show the areas and seasons most 
favorable to sid^marine operation. 

I'hese publications, like the sonar charts, have 
been something in the nature of an exjieriment. ft 
has not been certain throughout their development 
what information was needed most, nor were the 
available observations so complete as might be de¬ 
sired. The different Supplements have therefore 
been variable in cpiality and in manner of presenta¬ 
tion, and revisions are needed to make them more 


comprehensive and more uniform in general style 
and content. 

As in the case of prediction manuals, further de¬ 
velopments in subsurface warfare will necessitate 
occasional revisions of the Supplements. Statements 
previously made about changes in response to modi¬ 
fications of sonar ecpiipment apply ecpially to all 
these publications. Also it seems likely that subma¬ 
rines in the future may operate at greater depths than 
at present. This would require extensive revision of 
all charts and considerable changes in both strategy 
and tactics. 

24 BOTTOM SEDIMENT CHARTS 

Submarine warfare in coastal waters made it ap¬ 
parent early in AVorld Whar II that there was great 
need for study of the problems of echo ranging in 
shallow water and the development of rules for both 
prostdjmarine and antisidmiarine forces. Accord¬ 
ingly, the production of bottom sediment charts was 
begun, which showed the distribution of different 
kinds of sediments with notes on the way in which 
they might be expected to affect sound ranging. This 
work was begtin at a time when information on the 
acoustic (jualities of bottom sediments was very in¬ 
complete. Indeed the whole problem of echo ranging- 
in shallow water is a diffietdt one that has not yet 
been solved in all its details. 

The different kinds of bottom that were shown on 
the first charts were picked more or less arbitrarily. 
It was known that bottom reflection and scattering 
could materially increase or decrease echo ranges in 
shallow water and that in general a smooth hard bot¬ 
tom extended the range, while a rough bottom short¬ 
ened it. But the details of acoustical classification of 
these bottoms were not kno^vn, nor where the boun¬ 
daries should be drawn between them. A detailed 
geological map would obviously be too complex for 
the purpose at hand, therefore it was necessary to 
group the many types of bottom that occur under 
the sea into a few general classes. As it later turned 
out, the choices were fortunate and, as experimental 
data have accumulated, only slight modifications of 
the range predictions have become necessary for the 
six major bottom types now in use. 

In examining the effect of bottom structure on 
echo ranging, it is important first of all to consider 
the comjjosition of bottom sediments. It is the tex¬ 
ture of the sediments that largely determines how 


RESTRICl'ED 






BOTTOM SEDIMENT CHARTS 


15 


much of the sound that reaches the Ijottom will be 
absorbed and how much will be rellccted back into 
the water. The softer the bottom is, the more sound 
will be absorbed, d ims a soft mud liottom will not 
reflect enough sound to return an echo from a sub¬ 
marine, and echo ranges can be predicted, as in deep 
water, according to the refraction pattern. A sandy 
bottom, on the other hand, may reflect nearly all the 
sound, so that there will be little correlation between 
refraction and the range obtained. 

In the bottom sediment charts, the criterion estab¬ 
lished for estimating the relative firmness or softness 
of the bottom was grain size,’' as determined by me¬ 
chanical analysis. The classification has been reason¬ 
ably satisfactory from the acoustical standpi^int ex¬ 
cept in the case of mud, in which it is now apparent 
that texture alone is not an adequate criterion. Two 
mud deposits with the same particle size may behave 
very differently with regard to the absorption of 
sound, one absorbing it completely, the other not at 
all. It is suspected, although not proved, that mud 
which gives some extension of range by reflection 
contains considerable clay, which imparts firmness to 
the structure. Tests of the plasticity of critical de¬ 
posits are needed to determine this theory. Once the 
problem has been solved by a proper combination of 
acoustical tests and laboratory analysis, the classifi¬ 
cation can be altered accordingly. If the percentage 
of clay in the samples proves to be the answ^er, many 
such areas could probably be located by geological 
inference and this information placed on the charts. 

The construction of bottom sediment charts neces¬ 
sitated the synthesis of material from several sources 
—navigational charts, oceanographic surveys, acous¬ 
tical tests, and general geological knowledge of the 
way sediments are transported and how they are re- 

The size limits were set arbitrarily as follows: 

MUD — 90 per cent smaller than ().0()2 nun. 

SAND ,\ND MUD —Between 10 and 90 per cent smaller 
than 0.062 mm. 

SAND — Less than 10 per cent smaller than 0.062 mm and 
90 jrer cent smaller than 2.0 mm. 

STONY - Rounded or angular pieces of rock more than 
2.0 mm and less than 10 cm, which appear to rep¬ 
resent glacial drift or other transported material. 

ROCK — Rocks of a size greater than 10 cm or pieces bro¬ 
ken from rock ledges or wliere Itottom photo¬ 
graphs show projecting rocks or rock ledges. 

CORAL — Calcareous ma.sses of coral, algae, or other lime 
secreting organisms, as shown by samples or bot¬ 
tom photographs. 


laled to bottom topography. Perhaps the best way to 
understand how this was done is to review the history 
of the charts. 

Prior to the war, hundreds of bottom samples and 
cores in all types of sediments had been taken by the 
Scripps Institution of Oceanography and the Woods 
Hole Oceanographic Institution in the course of 
their work in submarine geology. Mechanical anal¬ 
yses had been made on most of this material, so that 
considerable information was available for the con¬ 
struction of charts that could be used for acoustical 
work in local areas. AVhere coverage by actual 
samples was not complete enough, additional ones 
were immediately taken. The first bottom charts of 
the east coast included the easterly end of Long 
Island Sound, Block Island Sound, the approaches 
to New York, and Massachusetts Bay. These were 
used in experimental sound ranging by the Colum¬ 
bia Lhiiversity Division of Whir Research at the U. S. 
Navy Underwater Sound Laboratory at New London 
and for ambient noise surveys off New York and in 
Block Island Sound. On the west coast, similar charts 
were made for the vicinity of San Diego and San 
Francisco Bay and were used by the University of 
California Division of War Research at the U. S. 
Navy Radio and Sound Laboratory, San Diego, for 
acoustical tests of the bottom. During these charting 
programs, close contact was maintained between the 
east and west coast groups so that the classification of 
sediments and the methods of chart construction 
woidd be comparable in every respect. 

Acoustical tests in areas such as the San Diego 
oiling, where bottom types varied widely in relatively 
short distances, required great care in demarcating 
bottom zones and keeping within the zones during 
tests. In some areas where abundant bottom samples 
showed sand, or sand and mud, dredging revealed the 
jiresence of scattered rock ledges, and in these areas 
the reverberation level was distinctly higher than in 
other zones where such ledges did not occur. Thus it 
was apparent that very careful sampling was recpiired 
for acoustical work on the bottom, and both dredge 
lines and bottom samples were needed along the 
course of projiosed sound field runs. 

In rocky areas, dredging often proved slow and 
laborious, and photographs of the bottom taken with 
an underwater camera" were found to be a better 
method. 1 hese photographs, of which exanqjles are 
shown in Chapter 9, were (juite adequate for the pur¬ 
pose when taken at closely spaced intervals, and often 


RESl'RICTED 









16 


METHODS FOR PREDICTING SONAR AND DIVING CONDITIONS 



revealed useful details of bottom topography that 
could not be determined by bottom samples. 

As soon as the preliminary program of charting 
and acoustical tests in local areas had progressed far 
enough to provide a scientilic basis for further work, 
a general charting jjrogram was begun. Outside the 
areas where samples were obtainable, it was neces¬ 
sary to depend principally on bottom information 
collected during government hydrographic surveys 
together with the small amount of material reported 
in various oceanographic investigations. The surveys 
varied greatly from place to place in detail and relia¬ 
bility, depending on the importance of the region to 
navigation and on the country that conducted the 
survey. In all these cases involving transfer of infor¬ 
mation from na\ igational charts to charts of sound 
ranging conditions, judgment needed to be exercised 
on two counts: first, to attempt to compensate for 
lack of adeejuate information on the na\'igational 
charts by judging the situation according to what is 
known about particidar areas that have been sur¬ 
veyed by more thorough methods; second, to simplify 
the charts by using in some cases a classification that 
is functionally correct from the sound ranging stand- 
jM)int although not necessarily accurate geologically. 


The methods used in this work will be described in 
detail in a later chapter. 

During the course of several years’ work most of 
the important strategic areas have been charted, in¬ 
cluding the coast of the United States, the Philip¬ 
pines, the Japanese Islands, the east coast of (diina, 
French Indo-China, Malaya, the eastern side of the 
Bay of Bengal, parts of the East Indies, and selected 
areas off the European and African coasts. The loca¬ 
tion of these charts is shown in Figure 3. 

I’he bottom sediment charts are used for range 
predictions and are incorporated in the classification 
schemes in the matuials. They are therefore an im- 
jiortant part of the general tactical considerations 
in\ol\ed in the sjjacing of vessels and the operation 
of sonar ecjuipment so as to obtain maximum effi¬ 
ciency. I'hey are equally important to sidjmarines in 
the lattei resjiect and can also be used in choosing 
favorable operating areas. It seems probable, how¬ 
ever, that the full value of these charts will not be 
realized until work now underway has been com¬ 
pleted. Fhis will include not only general improve¬ 
ment of the charts along the lines already discussed, 
but also modifications of sonar equipment intended 
specifically to improve performance in shallow water. 


KT^T^TE1)"\ 





















































































Chapter 3 

TRANSMISSION OF SOUND IN SEAWATER 


3 1 PROPAGATION OF SOUND WAVES 

oiJNi) wa\cs propagated in a homogeneous niedi- 
inn radiate outward from the source in straight 
lines. If the sound source is constructed so as to be di¬ 
rectional, then most of the sound will be projected in 
the form of a conical beam with its axis perpendic¬ 
ular to the face of the projector, dhis situation is 
shown diagrtunmatically in Figure 1, in which A, li, 
and G tire successi\e sound imjjidses (pings) which 
can be visuali/ed as sections of the cone that consti- 
ttites the highly directional and most intense part of 
the emitted sound. Sound of weaker intensity will be 
projected at greater angles as indicated by dotted 
lines in the figure and will return an echo from a 
target at close range as indicated Ijy the enlarged en- 
\elope of the sound imptdse at A, but the greater the 
range the smaller will be the angular width of the 
effective cone, which with standard Navy sonar gear 
is approximately 10 to 12 degrees wide at ranges 
greater than 800 yards. 

3.1.1 Factors Affecting Transmission 

Because of divergence, the intensity of sound de¬ 
creases with increasing distance from the sound 
source. Except for some small loss by absorption, we 
can assume that the total amount of energy in the 
sound pulse is the same at ranges A, B, and G. But as 
the cross sectional area of the beam increases, the 
amount of sound that will be intercepted and turned 
back by a target of unit size decreases. Since the in¬ 
crease in the diameter of the pulse is proportional to 
the range, the increase in its area is proportional to 


the stpiare of the range. It follows then that within 
any area of constant size within the sound pidsc, the 
intensity decrease will be inversely proportional to 
the stpiare of the range. 

Wdien the sound striking a target is scattered back, 
then the rellected sound ira\eling away from the 
target suffers intensity loss by di\ergence in the same 
way as does the outgoing sotnul pulse from the |)io- 
jector. Therefore, in passing from target to receixer 
the intensity of the echo decreases as the inxerse 
sc|uare of the range. Gombining these txvo losses, the 
total decrease in intensity between the outgoing 
sotmd jjulse and the echo that comes back is j)roj)or- 
tional to the inxerse fourth poxver of the range. For 
every tenfold increase in range, the intensity in the 
direct beam will be decreased to 1 MOO of its original 
\alue, and the echo intensity by the time it reaches 
the receixer xvill be redticed to 1 /10,000. Absorption 
of sound by the water makes this decrease in sotmd 
intensity exen more rapid, particularly at long 
ranges. All this explains why the sound of a ping at a 
range of 10,000 yards may be louder than an echo 
from a target at 1,000 yards, and xvhy echo ranges are 
distinctly limited by the poxver of the gear. It also 
makes it clear that a very great increase in sound out¬ 
put xvould be re(|uired to prodtice a moderate in¬ 
crease in echo ranges. 

I'he strength of an echo depends on the size of the 
target as xvell as on range. Thus a large vessel returns 
a stronger echo than a small one, and for a ship of any 
gixen size the beam aspect presents a better target 
than boxv or stern. 

M'hese are a fexv of the simple properties of sound 





















18 


TRANSMISSION OF SOUND IN SEA ^VATER 



Fk;i're 2. Diagraminalic draw iiif^ of outgoing ping sliowing eflect ol relraclii)ii. 


transmission in a homogeneous medinm. Horvever, 
the sea is far from homogeneous and, moreover, in 
echo ranging the source of the sound is quite near the 
surface, which complicates the situation by reflecting 
and scattering sound. I'hc physical characteristics of 
sea water—its temperature and salinity, the number 
and size of the particles suspended in it, and the dis¬ 
turbances of its surface—vary widely from place to 
place and from time to time. All these affect trans¬ 
mission, and underwater sound is therefore sidDjcct 
not only to expected divergence and absorption, hut 
also to \arying degrees of refraction, reflection, and 
scattering. 

Reflection 

Reflection occurs both from the ocean bottom and 
from the underside of the sea surface. If these surfaces 
are smooth, extension of range may result through 
reinforcement of the direct sound beam. If they are 
rough, sound cvill be scattered in all directions, and 
that ]jart which is returned to the location of the 
original sound source, thereby interfering with echo 
recognition, is known as reverberation. The bottom 
effects arc important enough to he regarded as the 
controlling factor in echo ranging wherever the 
depth of water is less than about 100 or 200 fathoms. 
Surface reflection and scattering are usually less sig¬ 
nificant in standard echo ranging. However, at short 
ranges (less than 1,000 yards) reverberation from the 
surface forms the principal background in echo rang¬ 
ing and may be the controlling factor in the detection 
of small objects whene\ er the sea is not calm. 


Refraction 

Variations in the temperature and salinity of sea 
water can profoundly affect sound transmission be¬ 
cause they j:)roduce variation in the speed of sound as 
it tra\ els from one jDoint to another, and this in turn 
causes refraction of sound waves. It was shown in the 
introductory chapters how discocery of this phenom¬ 
enon led to the clc\elopment of the hatJiythermo- 
grnph [BT] and accompanying prediction methods 
as instruments of naval warfare. At this time it is 
worth-while to discuss the principles of the simple 
refraction theory as it evas first developed, since it 
provides an adecpiate framework for the essential 
facts on underwater sound transmission and the way 
in which maximum echo and listening ranges are 
affected by tenijierature gradients. It shoidd be kept 
in mind, hotvever, that while the theory is in ajiproxi- 
mate agreement with the most important observed 
results, subse(|uent experiments have shown it to be 
an incomplete ex])lanation. Discussion of these later 
modifications of the refraction theory is beyond the 
scope of the present work but will be taken up in 
other volumes of the Summary Technical Report of 
Dir ision 6. 

Sound waves tra\el in straight lines onlv in a me¬ 
dium in which the speed is everywhere constant. In 
sea water the speed of sound generally \aries with 
depth. Siq)j)ose, for example, the speed increases with 
depth. In that case e\ery ray of the sound beam will 
be cuiAed toward the surface. The more rapid the 
change of speed with dejith, the more strongly the 
rays will be curved. This bending of the sound rays is 












PROPAGATION OF SOUND WAVES 


19 



called rclVaction. It will aid in understanding refrac¬ 
tion to imagine that the lower part of the beam, in 
the example just cited, keej^s getting slightly ahead 
of the upper part, because of the slight difference in 
speed in the water at the levels at which the two parts 
of the beam travel. It is a fundamental law of wave 
motion that the rays, which indicate the direction of 
travel, are always perpendicular to the wave fronts. 
A wave front is the surface occupied by the front of a 
sound signal at any instant. Because the lower part of 
the wave front keeps gaining on the upper, the beam 
will cur\'e upward. 

If the speed decreased with depth, the beam would 
bend downward. Refraction always causes a sound 
ray to shift its course toward the water in which the 
speed is lower. This condition is illustrated in exag¬ 
gerated form in Figure 2. 

\Tlocitv of .Sound 

The speed of sound in sea water depends on the 
temperature and composition of the water. In gen¬ 
eral these quantities vary both horizontally and ver¬ 
tically, but only the latter is of any great significance. 
W'hile horizontal changes in the speed ol sound will 
cause changes in the time required for a signal to 
travel between two points, this effect is small and 
never serious in echo ranging. But changes in velocity 
with depth, even slight ones due to warming of the 
stirface water on a l)right, calm day, dellect the sound 
beam IVom the horizontal plane and may cause it to 
o\ ershoot or undershoot the target. 

In echo ranging work, in which only the upper few 
hundred feet of water are involved, temperature is 


generally the most important factor causing varia¬ 
tions in sound velocity. The chemical content of sea 
water, of which salt is the major constituent and the 
only one which need be considered, is relatively uni¬ 
form in the open ocean and is therefore less impor¬ 
tant than temperature in determining sound veloc¬ 
ity. Furthermore, as will be shown later, in layers 
where vertical salinity gradients exist there is nearly 
always also a vertical temperature gradient. Another 
minor factor is pressure, which increases proportion¬ 
ally with depth. The effect of these three variables on 
the speed of sound is shown in Figure 3. It will be 
noted that they all change the \elocity in the same 
direction. An increase in temperature, salinity, or 
depth (pressure) causes an increase in the velocity of 
sound. 

Gradients 

In sound transmission the vertical velocity gra¬ 
dient is more important than the velocity itself, since 
it is the change in velocity with depth that determines 
how much refraction will take place. Idle velocity 
gradient is readily determined from the gradients of 
salinity and temperature. The salinity gradient is de¬ 
fined as the rate of increase of salinity with depth, in 
jiarts per thousand (‘’/oo) per foot. 1 he temperature 
gradient is the rate of increase of temperature with 
depth, in degrees Fahrenheit per foot, d hese gra¬ 
dients are therefore called positive if the ejuantity in 
(jiiestion increases with depth, negative if it decreases. 

It will be seen later that in the great majority of 
cases temperature gradients in the sea are zero or neg¬ 
ative. Moreover, except in certain localized areas 


RESTRICTED 







































20 


TRANSMISSION OF SOUND IN SEA WATER 


temperature gradients control the sound velocii) 
gradients. With a zero gradient in temperature 
(mixed layer), echo ranges are long because the sound 
rays arc very nearly straight, having only a slight up¬ 
ward cur\ature due to the pressure cllect. On the 
other hand, with a strong negati\e gradient near the 
surlace, echo ranges will be short because the sound 
beam is refracted sharply downward. In the ocean it 
is common to find a mixed layer overlying a negative 
temperature gradient. In such cases the echo range 
on a target in the mixed layer will be long, but the 
[jart of the sound beam that enters the negative gradi¬ 
ent will l)e refracted downward, residting in reduc¬ 
tion of range. 

Ihere are two phenomena which may cause re¬ 
duction of range by refraction. Whth a strong nega¬ 
tive temperature gradient from the surface down¬ 
ward, each ray of the sound beam curves down in a 
great arc, and beyond the horizontal limits of the 
beam is a so-called shadow zone into which no sound 
penetrates other than by scattering. An echo-ranging 
vessel will not be able to detect a target in the shadow 
zone, but as soon as the latter comes within the direct 
beam the echoes will come in loud and clear. This is 
the situation illustrated in Figure 2, in which the 
submarine at A is in the direct beam and the one at 
B is in the shadow zone. 

With slighter negative temperature gradients and 
generally with any gradient underlying a mixed 
layer, the shadow zone is not \ ery clearly defined. But 
downward refraction at any depth below projector 
level exaggerates the normal divergence of sound 
rays. And since the intensity of sound within the 
beam is in^er.sely proportional to the divergence of 
the sound beam, refraction under these conditions 
has the effect of reducing echo intensity and thereby 
reducing the range. Thus a target in the negati\e 
gradient beneath a mixed layer may be within the 
direct beam but still be undetectable because the 
echoes are too weak to be heard against the back¬ 
ground of reverberation and shijj’s noise. This is 
known as layer effect. 

Bottom Effects 

As stated previously, sound transmission in shal¬ 
low water is complicated by bottom effects, ^\dlerevcr 
the depth is greater than 200 fathoms, the bottom can 
be neglected in echo ranging. In contrast, where the 
water is less than 100 fathoms deep, the bottom sedi¬ 


ments frecjuently become the limiting factor in deter¬ 
mining maximum range. Between the 200-fathom 
contour and the lOO-fathom contour is a zone of un¬ 
certainty as far as the effect of the bottom is con¬ 
cerned. However, the bottom usually slopes so steeply 
l)etween these two contours that only a small fraction 
of the ocean area is in\ oh ed. 

^Vhat makes the situation partictdarly difficult in 
shallow water is that both the texture of the sedi¬ 
ments and the topography of the bottom are acousti¬ 
cally important. The bottom scatters some sound 
ba( k toward the source, gi^ ing rise to re\erberation 
above which the echo must be recognized, and it may 
also act as an efficient reflecting surface for extend¬ 
ing sound into areas where the intensity would other¬ 
wise be too low to return an echo. Thus a smooth, 
hard bottom can increase the maximum range over 
what woidd be ex})ected under the same refraction 
conditions in deep water, but a rough bottom may 
cause such loud rc\erbcration that the echo cannot 
be recognized. 

Variations in echo and reverberation intensity re¬ 
sulting from multiple reflections frequently result in 
the so-called skip distauce effect. Contact may first be 
established at relatively long ranges through bottom- 
reflected sound, but at medium ranges where the di¬ 
rect sound beam strikes the bottom, reverberation 
may be strong enough to mask the echo. Then at 
short ranges the echo level may be higher than the 
reverberation level, and contact will be regained. If 
the target is close to the bottom, skip distances are 
uncommon. Such a target will be struck by the sound 
Ijeani at approximately the same range at which the 
l)ottom is struck as well. Thus, maxima in echo 
strength may be expected at the same ranges at which 
the bottom reverberation will also be strong. If the 
reverberation is strong enough to mask the echo at 
one particular range, it will mask the echo at all 
longer ranges as well. AN'hether bottom reverberation 
is strong enough to mask an echo depends primarily 
on the bottom material, and also on the depth of the 
water, the strength of he echo, and the refraction pat¬ 
tern. 1 he range at which masking by rc\crberation 
is first likely to occur is ecpial to al)out four to six 
times the depth of the water, pro\’ided that strong 
negatixe temperature gradients are present, llie 
range over xvhich such masking may persist for a 
shallow target will vary, dej^ending in part on the 
sediment, but commonly it is about .500 to 700 yards. 






PROPAGATION OF SOUND WAVES 


21 


\\'heii the water is isothermal to the Itottom, the 
range at which masking I^y reverberation is first likely 
is more nearly ten times the water depth. 

Except over soft mnd bottoms, which absorb 
nearly all the sound reaching them, or tvhere there is 
strong upward relraction, layer cirect is absent or 
much reduced in shallow water because the low in¬ 
tensity parts of the direct sound field are more or less 
filled with bottom-reflected sound. 

O i HER Factors 

False echoes are relatively frctpient in shallow 
water, although well-trained operators should nearly 
always be able to classify them correctly. False echoes 
may be of several varieties: echoes from schools of 
fish, bottom irregidarities, or wrecks, and sharp local 
rises in the reverberation level under conditions of 
strong downward refraction. 

Fhc ambient noise level will vary in shallow water, 
not only because of noise of biological origin and 
noise from waves, but also depending on the prox¬ 
imity to the shore, especially if the surf is heavy. In 
addition, the noise level will depend somewhat on 
the reflecting qualities of the bottom. 

therefore, in general it can be concluded that 
sound conditions in coastal waters are highly variable 
and tend to be poor except over a smooth, sandy 
bottom. These factors have in general been favorable 
to submarines. Since shipping must converge off the 
main ports and since much cargo is carried coastwise, 
it is frequently possible for submarines to lie in wait 


in particular areas that are oceanograjjhically suit¬ 
able. Ihese advantages are partly offset by the fact 
that effccti\e air coverage is more easily maintained 
near land; nevertheless, sid^marine activity could 
continue in coastal waters, especially where favorable 
oceanographic factors coincide with a high density 
of shipping. It is apparent therefore that both pro¬ 
submarine and antisubmarine groups must recognize 
and understand the problems of range prediction in 
shallow water. 

These, then, are the major factors involved in the 
transmission of sound in sea water. The BT fills the 
obv ious need for an instrument to measure the tem¬ 
perature gradients in the water. Idle prediction 
manuals serve to convert this information into a 
form that is tactically usable. However, there always 
remains the question of how long the observed condi¬ 
tions will remain reasonabh constant, or to put it 
another way, how often BT readings will be required 
and what sound conditions arc likely to be the next 
day and the day after, lliese questions are partially 
answered by the sonar charts, but the latter are of lim¬ 
ited value in the same way that a chart of average 
weather conditions is limited in its value for predict¬ 
ing the weather tomorrow. Use of the charts and of 
BT predictions in general will therefore be much 
improved if it is tempered by judgments based on 
some understanding of physical oceanography and 
weather and the interrelations of both with sound 
conditions. The basic principles of this knowledge 
will be summarized in Parts 2 and 3. 


RESTRICTS 






Chapter 4 

SUBMARINE DIVING PROBLEMS 


INTRODUCTION 

I N THE chapters that follow there are numerous 
references to submarine diving problems in con¬ 
nection with particular oceanograjdtic conditions. Be¬ 
fore the development of the submarine bathythermo¬ 
graph [BT], the submariner had no method for de¬ 
termining cpiickly and easily the oceanographic 
characteristics of the waters in which he was operat¬ 
ing. Diving and maintaining trim at the desired 
depth were largely a matter of trial and error and 
were therefore costly in time and effort. The diving 
rules developed in connection with the BT have 
greatly simplified these operations. The whole sub¬ 
ject is covered in detail in \'olume 6B of Division 6. 
In the meantime the subject will be outlined broadly 
by way of introduction to oceanographic considera¬ 
tions discussed in other chapters of this volume. 

A submerged submarine has two methods of chang¬ 
ing depth, which may be used singly or together. 
First, it can change its buoyancy by regulating the 
amount of sea water in its ballast tanks. Flooding bal¬ 
last makes the submarine less buoyant so that it sinks; 
pumping ballast makes it more buoyant, and it rises. 
Second, the ele^'ation of the diving planes can be 
changed so that as the submarine moves through the 
water it planes up or down. A modification of this 
method is to ballast unecjually fore and aft. The hull 
then lies at an angle in the water, and the ])laning 
effect is produced without changing the diving 
planes. 

It is of the greatest importance in submarine oper¬ 
ation to be able to change depth efficiently. It often 
takes an appreciable amount of time to flood or 
pump the requisite amount of water. During offen¬ 
sive or defensive operations a delay of a few minutes 
caused by faulty judgment as to the correct ballast 
change may be costly. The noise involved in diving 
operations is also an important consideration when 
operating among enemy ships that may be maintain¬ 
ing a listening watch. 

With these considerations in mind, it is e\ident 
that optimum efficiency in diving operations means 
achieving the best possible balance between speed in 
completing the change in depth and ([uietness of 


OO 


operation throughout. It implies knowledge before¬ 
hand of what ballast changes will be needed and 
when, so that the submarine will not at any time be¬ 
come dangerously out of trim and require a sudden 
burst of speed or other noisy operation to keep it 
under control. 

4 2 DENSITY LAYERS 

Maintaining efficient di\ing operations would be 
simple if the buoyancy of sea water were everywhere 
uniform. Since it is not, the variations in density that 
occur make each dive a separate problem requiring 
slightly different tactics. I'hese are considered briefly 
below. 

If, for example, the sid^marine is in trim at peri¬ 
scope depth, its overall density is approximately the 
same as that of the surrounding sea water. Conse¬ 
quently, it has no great tendency either to rise or sink, 
and such small movements as occur are readily cor¬ 
rected with slight changes in the angle of the diving 
planes. As the vessel travels at periscope dej)th it may 
move into water of greater or less density. An increase 
in temperature makes the water expand so that it is 
lighter. 4die density also depends on its salt content 
(see Section 5.1). Such changes in density require re¬ 
ballasting to bring the submarine back into trim. 
However, these lateral density changes are relatively 
slight in most cases, and it requires no great effort to 
keep the vessel on a horizontal course. 

If a submarine in trim at periscope dcq)th dives in 
water of uniform density, it gradually gets out of 
trim because the increasing pressure at greater depths 
compresses the hull, making the submarine less buoy¬ 
ant. Therefore under these conditions a sid)marine 
must pump ballast during the dive in order to main¬ 
tain trim. The amount of rvater to be jmmped out 
depends on the size of the sid^marine and on its com- 
pressibility, the latter varying with the type of con¬ 
struction and, to a lesser extent, with individual 
vessels. 

The case abo\e is one of the rarer examples of a 
diving o])eration, because the water is not often of 
uniform density down to the maximum depth of 
submarine operation. 


RESTRICTED 





TEMPERATURE VAREATIONS 


23 


DEGREES FAHRENHEIT 


30 40 50 60 70 80 90 



Figure 1. Submarine bathythenno^rain showing eflecT of 
temperature gratlients on ballasting operations. 


DEGREES FAHRENHEIT 



30 40 50 60 70 80 90 


Fku're 2. Submarine bathythermogram showing diving 
operations iii a mived layer with an underlying negative 
gradient. 


43 TEMPERATURE VARIATIONS 

It was pointed out in the introduction that the 
temperature may decrease from the surface down¬ 
ward or in the water underlying a mixed surface 
layer. With decreasing temperature the density 
would increase downward. The sea may be thought 
of as a series of horizontal layers one below another, 
in which the density is either uniform or increases 
downward l)y a greater or lesser amount. 

A layer of increasing density gives a diving subma¬ 
rine more support and tends to counteract the com¬ 
pression effect. It is conceivable that the two effects 
may just balance, so that a diving submarine will re¬ 
main in trim all the way down. On the submarine 
BT card (Figure 1) are printed isoliallast lines, which 
show the amount of temperature change with depth 
that will, by its effect on the density of the water, 
exactly balance the comjiression effect of a subma¬ 
rine of the type for which the card was prepared. In 
any layer where the temperature-depth trace parallels 
the isoballast lines, the diving submarine will remain 
in the same state of trim throughout the layer. If the 
temperature is more nearly uniform, so that the trace 
crosses the isoballast lines toward the right, a diving 
submarine will get heavier and will have to pump 
ballast to regain trim. In a strong gradient that 
crosses toward the left, it will be light and will have 
to flood ballast. 

As is already apparent from jjrevious discussions, 
it is common to find temperature conditions in the 
sea of the kind shown in Figure 2, in which there is 
a surface layer of mixed water and an underlying 
layer with a sharp decrease in temperature. Suppose 
a submarine is in trim at periscope depth (Position 


A) and makes a dive with no ballast changes. As it 
goes through the mixed layer it gets heavier and 
sinks more rapidly. But the temperature gradient 
below gives it more ljuoyancy again, and it finally 
comes to trim at Position B, where the temperature 
trace intersects the same isoballast line that passed 
through Position A. This is an examjde of a very 
quick aud efficient dive that makes full use of a 
knowledge of the temperature conditions. It would 
be much less efficient to dive in trim, pumping while 
in the mixed layer and flooding again below it. How¬ 
ever, if there were no temperature gradient below 
the mixed layer, it would be more efficient as well as 
safer to remain more or less in trim all the way down. 
Thus, the correct diving procedure depends on 
knowledge of the vertical temperature structure of 
the water. The example described above is unusually 
simple since it is not commonly possible to dive with¬ 
out making any ballast changes at all. Nevertheless, 
the general principle holds in almost any case, that 
it is possible to dive more efficiently in water of 
known temperature structure than in an unknown 
situation because when the diving officer knows the 
total amount of ballast change that will be needed, 
he can make the proper adjustments at an even rate 
throngh the entire operation and will not be stopped 
by a sharp density gradient or forced to increase the 
noise output of the submarine in the effort to get 
through the layer. The time thus saved during a dive 
may be as much as 10 or 15 minutes. 

In order to obtain proper knowledge of the tem¬ 
perature structure of the water, it is necessary for the 
submarine to make fre([uent exjdoratory dives. Den¬ 
sity conditions are best dealt with in an emergency 


RE.STRICTED 






















































































































































24 


SUBMARINE DIVING PROBLEMS 


il a recent B'l’ record is availaljle. How often such 
dives are needed depends on the variability of tem¬ 
perature conditions where tite submarine is operat¬ 
ing, and for this purpose sonar charts and Subma¬ 
rine Supplements pro\ ide the sidmiariner with infor¬ 
mation about local variability. In olfensive and de¬ 
fensive operations, however, this information about 
a\erage layer depth is obviously less useful than the 
more complete and specific knowledge obtained by a 
recent exjjloratory di\e. 

I'he use of BT records for the control of diving 
makes no allowance for vertical density changes due 
to salinity structure, riiroughout the greater jjart of 
the deep oceanic regions \ariability of salinity is an 
insignificant factor, but there are local areas, partic- 
idarly in shallow water, where it is important. In 
such places temperature records must be used with 
discretion and supplemented where possible by sal¬ 
inity data in the Submarine Supplements or by gen¬ 
eral oceanographic knowledge of the kind described 
in sid)sequent chapters and dealt with more specifi¬ 
cally in the volume on diving control pre\'iously re¬ 
ferred to. More adecpiate information can be ob¬ 
tained by the use of the salinity-compensated BT. 
Whether or not it will be useful enough to justify 
installing such a complicated and bidky instrument 
will perhaps depend on the location of future stra¬ 
tegic areas. 

Further reference to the salinity-compensated BT 


may be found in Section 1.3 on “I’he Bathythermo- 
gra])h for Submarines.” 

4 1 PROGRESS OF FOREIGN NATIONS 

It is not known whether foreign nations have de¬ 
veloped diving control to the same degree as our own 
Na\ y, but it can at least be assumed that they under¬ 
stand the ad\antage of making use of density layers 
and have charts of a\cragc conditions similar to those 
in the Supplements. This much, as stated pre\ iously, 
has been determined from a captured German sub¬ 
marine, and the usefulness of density layers is too 
obvious to escape any sid)mariner. For them it is a 
fortunate coincidence that from both the acoustic 
and diving standpoints density layers provide the 
best possible protection in evasion. The so-called 
‘‘layer effect” has been mentioned, which reduces 
the echo and listening ranges on a submarine sub¬ 
merged well below the top of a density layer. It is also 
apparent that a submarine in the middle of such a 
layer recpiires little effort to maintain constant depth. 
If it rises it will be in water of less buoyancy and will 
tend to sink again. If it goes deeper it will encounter 
more buoyant water. Hence, it is not only easy to 
maintain (piiet operation, creeping, or balancing 
with the motor stojjped, but also it is easier to main¬ 
tain control of the ship during a depth-charge attack. 


RESTRICTED 






PART II 

TEMPERATURE AND SALINITY OF OCEAN WATERS 



• • • I 





[A 




Li 


I *• /s 









-t ; 


•f 


f' 


•rs 




1 


« -fyK- 

) 






Chapter 5 

THE BASIC VERTICAL THERMAL STRUCTURE 

OF THE OCEANS 


5 • THE PRIMARY SUBDIVISIONS 

A lthough in this report we are concerned only 
with the pliysical characteristics of the water 
down to the greatest depth in which a submarine can 
operate, it is advisable at the outset to consider brielly 
the vertical structure of the water column as a whole. 
I'he main ocean basins average some 2,500 fathoms 
(15,000 feet) in depth. Until recently sidiinarincs 
seldom descended to depths greater than 500 feet, but 
the lower limit may now be approaching twice this 
depth. Nevertheless, in the open ocean even 1,000 
feet is only a small fraction of the depth of the whole 
water column. 

From many standpoints, the upper one or two 
thousand feet is the most interesting part of the 
ocean. It is the part that is most affected by winds and 
weather, by seasonal changes in temperature, and by 
geographical variations in climate. "Fhe surface 
waters of the ocean are therefore highly variable. 
This variability presents many problems of joint in¬ 
terest to the science of oceanography and to the prac¬ 
tical applications of oceanography to subsurface 
warfare. 

By contrast the deep waters of the ocean are static. 
I'he most violent storms have little effect at depths 
greater than about 1,500 feet. Seasonal and geograph¬ 
ical variations are slight. But speaking in the absolute 
sense, no part of the ocean is completely static. AVater 
movement in the great depths takes place as a very 
slow drift of a large mass of water, in contrast to the 
more rapid and localized surface currents, but large 
volumes of water are transported by this means. Such 
movements are directly related to interchanges be¬ 
tween surface and deep water, which in turn play a 
part in determining the temperature pattern of the 
ujjjjer water. Therefore, it is hardly possible to un¬ 
derstand either vertical temperature structure or 
oceanic circulation without considering to some ex¬ 
tent the ocean as a whole. 

I'he basic thermal structure of the ocean is illus¬ 
trated in its simplest form in Figure 1, which is typ¬ 
ical of winter conditions in mid-latitndes. It is essen¬ 
tially a three-layered system: a relatively warm and 


TEMPERATURE 



Fi(;crf. I. Jia.sic thermal structure of the ocean—typical 
w inter conditions in mid-latitudes. 


shallow surface layer which has very nearly the same 
temperature as the air above it, and which is stirred 
by the wind so that the temperature changes little 
with depth; a very deep mass of much colder water in 
which temj)crature decreases very slightly and uni¬ 
formly with depth; and a third layer of transition 
between, known as the main thermocline. The term 
thermocline in oceanography is used for any layer in 
which temperature decreases markedly with depth. 
T his is a stable situation, that is to say, density also 
increases with depth. 

The concept of stability as a function of the den¬ 
sity appears frecpiently in any discussion of ocean¬ 
ography, and it is important to understand just what 
is iinolved. The density of sea water is dependent on 
its temperature, salt content, and the pressure of the 


RESTRICTED 


27 








28 


THE BASIC VERTICAL THERMAL STRUCTURE OF THE OCEANS 


DENSITY 



Figure 2. Efrect of teinperature, salinity, and pressure on 
density. 


surrounding water. Density increases when the sal¬ 
inity or pressure increases, but it decreases when the 
water expands with increasing temperature. \Vhen 
these properties arc known, the density can be deter¬ 
mined readily from standard tables such as those of 
Knudsen.^ Graphically the relationship can be repre¬ 
sented as in Figure 2. It can be seen that the effect of 
pressure is slight, and it usually can be neglected in 
oceanographic studies. Temperature and salinity are 
the factors ordinarily considered, and in most cases 
temperature is more important. 

W'ater masses of different density tend to arrange 
themselves in more or less horizontal strata with the 
lightest water at the surface and the most dense water 
at the bottom. This is a stable situation since work is 
recjuired by wind or forces producing turbulence in 


order to depress the light water or raise the heavier 
sufficiently to mix them to homogeneity. The effec- 
ti\’eness of the resistance of stable layers to vertical 
turbulence is indicated by the form of the tempera¬ 
ture curve in Figure 1, in which the depth of the 
mixed layer is a relati\ely small part of the whole 
water column. 

Continuing the discussion of the simple, three¬ 
layered winter ocean, Figtire 3 shows a diagrammatic 
sketch of a north-south profile through the North 
Atlantic. The relati\ely warm stirface layer is roughly 
lens-shaped in profile, being deepest in mid-latitudes. 
The thermocline, too, is deepest and thickest in mid¬ 
latitudes, and it intercepts the surface in a narrow 
band, just beyond the poleward limit of the warm 
surface layer. In latitudes higher than about 50° 
the entire water column is relatively cold, and this 
water is contintious with the cold deep layer that 
underlies the thermocline further south. 

To a certain degree, the distribution of the three 
primary layers is in agreement with the previous 
statement that waters of different density tend to 
form stable horizontal layers. However, it is also evi¬ 
dent that the layers tlo not achieve complete stability, 
else they woidd l)e found at all latitudes from the 
ecjuator to the j)oles and with uniform thickness. One 
of the main reasons for this lack of uniformity is the 
variation in tem):)erature and amount of solar radia¬ 
tion at different latitudes. In the tropics the water is 
being heated. As it decreases in density it e.xpands 
:md sjM'eads northward and southward along the stir 



LATITUDE 

60° 50° 40° 30° 2 0° 10° 0° 


1000 - 


uj 2000- 


3000- 


4000- 


5000- 


deep WATER 


Figure 3. Norih-soutli projection of the simple three-layered ocean in winter. 
























































THE PRIMARY SUBDIVISIONS 


29 


face, gradually cooling as it approaches high lati¬ 
tudes. As the warm surface water is drained off from 
the tropics it is replaced by underlying cold water 
that flows in underneath from the polar regions. 
Thus it is apparent that the deep water layer of all 
the oceans is idtimately derived from high latitudes 
and has much the same characteristics of salinity and 
temperature as the surface waters of those regions. 

At a latitude of about 50° North is a zone, pic¬ 
tured diagrammatically in Figure 3, where there is 
a considerable amount of mixing between arctic 
water and the warmer water from the south. It is a 
confused region, hydrographically speaking, with 
much eddy movement between masses of water of 
contrasting temperature and salinity, so that some¬ 
times the warmer water near the surface overlies the 
colder water and sometimes the colder water overlies 
the warmer water. The surface water carried to this 
region from the tropics is more saline than the polar 
water. As it gets colder in high latitudes it becomes 
dense enough to sink and become part of the main 
thermocline. At the point of sinking the surrounding 
surface water converges to take its place. Because of 
this characteristic, the region around 50° North is 
called the subarctic comiergencc (a similar zone in 
the .Southern Hemisphere is known as [he subanlarc- 
tic convergence). In the thermocline, as in the deep 
layer, there is a slow drift of water from its point of 
origin in high latitudes toward the tropics. 

The basic vertical temperature structure and the 
circulation of the ocean have been described as phe¬ 
nomena of heating and cooling and of water move¬ 
ments produced by the,se changes in density. It will 
be shown that this basic picture is modified by a num¬ 
ber of factors, in particular by the winds, by the 
earth’s rotation, by the configuration of the ocean 
basins, and by the .seasons. These factors will be con¬ 
sidered further in the appropriate jdaces. In the 
meantime, the review of the temperature structure is 
continued with a summary of practical implications 
and an examination of some of the temperature ob- 
ser\ ations that have been made. 

From the standpoint of subsurface warfare the 
aforementioned generalizations concerning the dis¬ 
tribution of the three primary thermal layers may 
be roughly summarized as follows: 

1. In winter beyond a latitude of about 50° in 
mid-ocean, and again in mid-latitudes, the water 
will be virtually isothermal down to the lower limit 
of submarine operation. 


2. The main thermocline approaches the surface in 
the equatorial region, becoming shallow enough to 
affect submarine operations by shortening the range 
on a submarine that is submerged below the isother¬ 
mal layer. The same is true near the edge of surface 
currents, such as the Gulf .Stream, that transport 
tropical water to higher latitudes. The thickening of 
the mixed layer in the central basins of the oceans 
and the thinning at the periphery are largely the re¬ 
sult of the wind systems of the earth, which produce 
surface currents in the ocean, driving the water in a 
great eddy in mid-latitudes in each ocean and tend¬ 
ing to concentrate the warm surface layer in mid¬ 
ocean. The effect of these wind-driven eddies is to im- 
pro\e sound ranges in the middle of the basins and 
reduce them at the periphery. 

3. In the subpolar convergences, where the warm 
water and the cold water are in close contact, the con¬ 
ditions arc variable. .Sometimes positive temperature 
gradients, sometimes negative temperature gi adients 
are encountered. 

Following this sketch of the vertical temperature 
distribution in a somewhat diagrammatic and ideal¬ 
ized ocean is a sample of the observations from which 
this simple picture has been derived. The standard 
instrument of physical oceanography has been the 
deep-sea reversing thermometer. Pairs of these instru¬ 
ments are usually lowered in series on a wire cable, 
the thermometers being attached to a frame known 
as a xuater-bottle. The mechanism is tripped by a 
mes.scnger sent down on the cable, which causes the 
thermometers to turn ujxside down, breaking the 
mercury column so that it records the temperature at 
the depth of reversal. .At the same time the bottle 
clo.ses, securing a sample of water from the same 
depth. I'hus, an oceanographic station, as it is called, 
consists of pairs of temperature readings, usually at 
aj)proximately 100-mctcr (330 feet) depth intervals 
and a water sample from each depth which is ana¬ 
lyzed for salinity and often for various dissolved sid)- 
stanccs as well. 

By using the deep-sea reversing thermometers in 
pairs it is po.ssible to overcome the difficulty that is 
caused by the current or the drift of the vessel which 
ordinarily prevent the wire supporting the instru¬ 
ment from hanging vertically. In each pair of ther¬ 
mometers one is protected by a glass case from the 
jjressure of the water. Because of the compressibility 
of the glass of the unprotected instrument, it will 
record a somewhat higher temperature than the pro- 


j^RE.STRICTED ^ 








THE BASIC VERTICAL THERMAL STRUCTURE OF THE OCEANS 


;i() 


35° 40° 45° 50° 



Figure t. Temperatiire-ilepth curves for liigh lalitudes 
in wiiuer. 

tectecl instriiiiiulU with which it is paired. From the 
ditferences in reading of the two instruments the 
depth can be caleulatcd with a very satisfactory de¬ 
gree of acctn acy. 

When good cjuality deep-sea reversing thermom¬ 
eters are used at sufficiently close dejtth interv als and 
the depths have been corrected for the changing 
angle of the stipporting wire cable, a smooth curve 
joining the observed points on a tcmjjcraturc-depth 
plot gives a reliable picture of the major features of 
the vertical thermal struettire at the station in cpies- 
tion. Unforttmately, in the past it has not always 
been possible to use reversing thermometers at suffi¬ 
ciently close depth intervals, especially near the 
surface. 

W'hilc the halliythcrtnograph [B'F] has very obvi¬ 
ous achantages, in its standard form it records the 
tem})erature clown to a depth of only 150 feet. Fur¬ 
thermore, the horizontal distribution of BT observa¬ 
tions is as yet inadeejuate. Therefore, many of the 
temperature-depth curves given below are primarily 
based on reversing thermometer observ ations, but arc 



Fita'RE 5. Teniperature-depth curves for luid-lalitudes 
ill winter. 


SO drawn as to agree nc;ir the surface with the avail¬ 
able B'F data from the area in c[tiestion. 

^ High Latitudes 

Unfortunately very few data are available from 
high latitudes in midwinter. Some of these are plot¬ 
ted in Figure 4 and show that the water coltmin is 
indeed essentially iscjithermal. Surface temperatures 
close to 52 F are frecpiently encountered in the open 
ocean, and even lower close to the land. Btu what 
lorms the bottom water in low latittides is not the 
coldest water produced each winter near the surface 
in high latitudes, btit the densest. This is water which 
has a salinity close to 51.8 and water of such a 
salinity is not found at the stn face over wide areas in 
winter where the temperature falls to much below 
55.6 F. 

Mid-latitudes 

Many winter stations have been occupied near the 
centers of the great wind-driven eddies. A selection 


RESTRICTED 





















THE PRIMARY SUBDIVISIONS 


31 



Figure 6. Temperature-depth curves lor low latitudes iu 
winter. 


i.s shown in Figure 5. It will be seen that the relatively 
warm, isothermal surface layer extends down to 
about 1,000 or 1,500 feet. I’he main thermocline, in 
which temperature decreases by about 23 F occupies 
the depths between approximately 1,000 feet and 
3,600 feet, while below in the deep water tempera¬ 


ture decreases at a fairly uniform rate of about 1 F 
per 1,000 feet. 

Tropics 

Figure 6 shows several stations from low latitudes. 
It will be seen that in the tropics the lower limit of 
the main thermocline is at about 2,100 feet, some 
1,500 feet less than in mid-latitudes and that it ex¬ 
tends up to within about 300 feet of the surface. Be¬ 
cause of the higher surface temperatures the total 
temperature drop across the main thermocline is 
about 15 F greater than in mid-latitudes. The rate of 
decrease of temperature with depth is also much 
greater, especially in the upper third of the main 
thermocline. As pointed out above, because of the 
small seasonal range in surface temperature in the 
tropics, summer observations would show much the 
same temperature-depth distribution. 

5.1.4 North-South Temperature-Depth 
Profile in Mid-Ocean 

Unfortunately it is impossible to show the thermal 
structure of any of the oceans in north-south profile 
based on midwinter oceanographic stations alone. It 
is necessary to combine stations made at different 
seasons of the year and to rely largely on BT data to 
reconstruct the surface layer. This is justified because 
below the depth of wind-stirring, seasonal tempera¬ 
ture changes are relatively small and result mainly 
from seasonal changes in the strength of the main 
currents which only slightly alter the average depth 
of the main thermocline. 

In Figure 7 the available data from the North and 


SOUTH LATITUDE 
50 " 40 ° 30 ° 20 ' 


NORTH LATITUDE 
20 ° 30 ° 40 ° 50 ° 



Figure 7. North-south temperature-depth profile in mid-Atlantic in winter. 


RE.SFRICTED 


J 



























32 


THE BASIC VERTICAL THERMAL STRUCTURE OF THE OCEANS 


South Alantic have been comltined into a mid-ocean 
jiiofilc. In both hemispheres winter conditions are 
shown. In other words, south of the equator the tem¬ 
peratures near the surface are typical of the Jtily- 
August period. It will be seen that the Southern 
Hcmisj)here half of the section is essentially a mirror 
image of the northern half and that both are in agree¬ 
ment with the more diagrammatic Figure 3, discussed 
above. 

Such attempts as have been made to construct 
similar north-south profiles of the other oceans all 
show the same basic sidjdivisions: a deep mass of rel¬ 
atively cold water, having but a slight and nearly 
uniform decrease of temperature with depth; a main 
thcrmoclinc at mid-depths, except in high latitudes; 
and above the main thcrmocline in winter a layer of 
relatively warm water of varying thickness in which 
there is virtually no temperature change with depth. 

Geographical Variations of the 
Primary Layers 

Hccausc of the difficidiy of representing the ocean 
in a three-dimensional diagram, it is necessary to rc- 
soi t to indirect methods in order to picture the hori¬ 
zontal and \'eriical dintensions of the three jirimary 
layers. 

I'hc difference between the minimum (winter) sur¬ 
face tcnqjcrature and the temperature at a dej)ih of 
600 feet is shown in Figure 8. In ctpiatorial regions 
the top of the main thcrmocline extends well aliovc 
the 600-foot level, so that the temperature difler- 
ence between the surface and this depth is relatively 
large. Northward and southward the difference be¬ 
comes much less as the surface layer thickens and the 
thermocline comes to lie at a greater dc])th. Beyond 
the jioleward limits of the main thermocline there is 
little tcnijicraturc change with dcj:)th. Hence the zero 
line marks off ajjproximately the limits of the main 
thcrmoclinc and the position of the sid^polar con¬ 
vergences previously mentioned. 

Some idea of the geographical variations of the 
warm surface layer can also be obtained from the 
layer dej)th on the sonar charts. I’hcse are repro¬ 
duced in F'igure 9, which shows the average layer 
depth during December, January, and February, and 
in Figure 10 which is for June, July, and August. A 
precise definition of layer depth is difficult, but it is 
usually the depth to the most prominent break in the 


temperature-depth ctirvc. This is commonly at the 
top of the seasonal thermocline, if present; otherwise, 
at the top of the main thermocline. I'he charts for 
the winter season (Figure 9, Northern Hemisphere; 
F'igure 10, Southern) show the geographical details of 
some of the feattires that have already been men¬ 
tioned: a mixed layer of 300 feet or more in each of 
the central basins in mid-latitudes, a shallower layer 
toward the continents on each side and toward the 
equator. Howe\cr, as there is no marked change in 
layer depth in the region of the stdipolar converg¬ 
ences, the sonar charts show little c\idence of this 
phenomenon. 

In the part of the charts depicting the summer 
season layer depths are, of course, shallower because 
of seasonal warming of the surface waters, and the 
charts no longer show the position of the main ther- 
modine. Discussion of this j)art of the charts is post¬ 
poned until the seasonal temperature cycle has been 
described. 

The lower part of the main thermocline and the 
underlying deep water are of less interest from the 
practical standpoint than the surface layer. How- 
e\er, in a few places they come close enough to the 
surface so that a submarine can operate in them. As 
was shown in previous figures, there is a gradual 
change in the slope of the temperature-depth curve 
between the middle of the main thermocline and the 
deep water. At no place is there a sharp break where 
the one ends and the other begins. But for practical 
purposes the lower limit of the main thermocline can 
be set at a temperature of about 11 F, below which 
further decreases in temjjerature with depth are very 
slight. 

In etpiatorial regions the depth of the 41-degree 
isotherm is generally about 2,000 feet, although in a 
few areas it is nuuh shallower. Along the west coasts 
of the continents, currents set iqi by the trade winds 
transport surface water away from the land, thin¬ 
ning out the warm surface layer so that the thermo¬ 
cline and deep water layer are drawn iqi near the 
surface. This process is known as tipxueUing, and 
where it occurs to a pronounced degree, the 41-degree 
isotherm may rise to within 300 feet of the surface. 

I’o the north and south of the etpiator the 41- 
degree isotherm slopes downward, reaching its great¬ 
est depths, 3,600 to 4,000 feet, in the central basins in 
mid-latitudes. It shoals toward the periphery of the 
wind-driven eddies and comes to the stirface in 



higher latitudes in the sidipolar convergences. 






Figure 8. World chart showing minimum temperature at the surface minus the temperature at 600 feet, 


/ 

/ 

/ 

-in 



^— ‘ 


's*. 

\ 

-. 


"T- rt ft f*i n n n. J- 

-- 

iMnnimMiiro: 6 






























































































































































































































Figure 9. World chart showing average layer depth during December, January, and February. 









































































































































































































±rccct 








Figure 10. World chart showing average layer depth during June, July, and August. 




































































































































































































DIURNAL AND SEASONAL CHANGES 


33 


5 2 DIURNAL AND SEASONAL CHANGES 
® ‘ * Heat Exchange at the Sea Surface 

Since the leinperaiure of tlic earth as a whole is not 
changing apprecial^ly, it is clear that the total 
amount of lieat received Irom the sun must exactly 
equal the amount lost by radiation back into space. 
But though the total heat budget must always bal¬ 
ance, the solar radiation and the loss of heat by radia¬ 
tion from the surface (called back radiation) at any 
one place and time seldom do. M any given j)lace 
and time the surtace of the planet is either warming 
or cooling. Local \ariations notwithstanding, the in¬ 
tensity of solar radiation is largely a function of lati¬ 
tude and season, and the difference in temperature 
between the tropics and the poles woidd be much 
greater than it is if it were not for the relati\ely free 
circidation of the atmosphere and the oceans. This 
causes the large-scale transfer of water mentioned 
earlier, and locally it gives rise to convection currents 
and largely controls the immediate details of the sur¬ 
face layer. 

Since the surface of the sea is an interface between 
air and water, the heat exchange involves other fac¬ 
tors besides radiation. Conduction may be in either 
direction depending on whether air or water is 
warmer. E\aporation (and its opposite, condensa¬ 
tion) and j)recipitation affect not only temperature 
but salinity as well, and so influence the density of the 
water in two ways. In the case of evaporation, the two 
factors always work together; evaporation makes the 
water denser both by cooling it and by increasing its 
salinity. Precipitation, on the other hand, always 
makes the tvater fresher, and so tends to make it less 
dense, but it may either warm or cool it according to 
the temperature of the rain. Figure 11 shows a bathy- 
thermogram taken a few’ hours after a heavy rain. 
Here the rain was evidently colder than the sea and 
there is a strong positive temperature gradient be- 
tw’een the layer of cool, fresh, rain-diluted w’ater and 
the warm, salty water underneath. 

Of the processes just mentioned all but solar radia¬ 
tion influence directly only the surface film. In the 
absence of mixing, only conduction and diffusion, 
which arc relatixely ineffective, can carry downward 
the changed surface conditions. Solar radiation, how’- 
c\er, has some powx*r of jjenetration. The depth of 
jienetration varies wdth the wave length of the radia¬ 
tion, being greatest in the visible part of the spec- 



Figi'ri; II. Bath\thermogram showing positive tempera¬ 
ture gradient at the surface due to rain. 


trum and much less for infrared and ultraviolet rays. 
In general, for any water of uniform transparency 
the rate of absorption for any particular W'ave length 
w’ill be constant; that is, if a certain per cent of the 
entire amount of energy of that wave length is ab¬ 
sorbed in the first inch of depth, the same per cent of 
the remaining energy w’ill be absorbed in the second 
inch, and so on. This characteristic is illustrated in 
Figure 12, in w’hich the percentage of light pene¬ 
tration is plotted on a logarithmic scale against 
depth. I'he fact that the data for each kind of w’atcr 
follow’ very nearly a straight line is indicative of a 
constant rate of absorption. Light penetration de¬ 
pends to a large degree on the transparency of the 
water. Pure water is highly transparent, but the sea 
is more or less turbid; it contains opaque particles in 
suspension that absorb almost all the radiation that 
strikes them. Hence Figure 12 show's a greater depth 
of light penetration in the clear waters of the central 
.Atlantic than in the more turbid coastal w’aters. 

Since heat is produced when radiant energy is ab¬ 
sorbed, the data on light absorption give an idea of 
the general characteristics of surface heating. How¬ 
ever, it is important to note that by far the most sig¬ 
nificant part of surface heating is accomplished not 
by visible light but by infrared radiation, which has 
less power of penetration and a much higher absorp¬ 
tion rate. Thus, if absorjjtion cur\es for infrared 
radiation w'ere shown in Figure 12 (w’hich is for 
visible light only) they would be much more nearly 
horizontal, and w’ould cross, say, the 50 per cent line 
only a few feet below the surface even in the clearest 
w’ater. Further, it is obvious that the absorption curve 


RESTRICTED ^ 

—- 















































































































































































34 


THE BASIC VERTICAL THERMAL STRUCTURE OF THE OCEANS 



FiCiURE 12. Absorption of visible lij>ht l)y sea water. 


PERCENTAGE OF TOTAL INCIDENT SOLAR ENERGY 



Fku're 13. .\bsorptioii by sea water of total incident solar 
eneiy^y. 


for total incident solar energy will not be linear, since 
it combines all wave lengths present. Figure 13 shows 
the absorption of total solar energy as compitted lor 
several different kinds of ocean water. In the ab,sence 
of con\'ection or turbulent mixing the temperature 
gradient produced by solar heating can be expected 
approximately to follow sitch an absorption cur\e. 
d'he heating will be greatest at the surface and will 
decrease rapidly with depth. The maximum depth 
at which a measurable increa.se in water temperature 
occurs will be perhaps 10 to 30 feet. 

This situation is nearly always modified by vertical 
mixing which tends to distribute the heated surface 
water downward. I'hus the obser\ed temperature 
increase at the surface is less than would be jjredicted 
according to radiation measurements, and the in¬ 
crease at lower depths is greater. Indeed in 70 or 80 
jrer cent of the observations that have been made, 
\ertical mixing has been sufficient to distribute heat 
completely uniformly, so that there is no observable 
temperature gradient. 

Diurnal ^\TRIM1NG 

The daily variation in the temperature of the 
superficial layer of water that occurs as a result of 
heating during the day and cooling at night is known 
as diurnal warming. An examjile of the kind of di¬ 
urnal warming that occurs during calm weather is 


shown by the series of bathythermograms in Figure 
14. In the early morning the temperature of the water 
was essentially uniform. During the middle of the 
morning surface heating became strong enough to 
create a slight negati\e gradient in the upper few 
feet. Fleating progressed during the day, and the 
negati\e gradient reached its maximum de\elop- 
ment in the late afternoon. Then cooling processes 
and vertical mixing gained the upper hand, produc¬ 
ing a shallow mixed layer which ajiparently deep¬ 
ened during the night until the negative gradient 
was destroyed. Early the following morning the water 
column was again e.ssentially isothermal except for a 
slight positive temperature gradient at the surface. 
The latter occasionally haj^pens when surface cool¬ 
ing is pronounced and the weather is calm. 

Figure 15 shows a similar series of bathythermo¬ 
grams taken during a 2-day period. I’he general fea¬ 
tures of diurnal warming are similar to those previ¬ 
ously shown, but the figure ser\es to illustrate minor 
\ariations in the form of the negati\e gradient and 
the depth of heating such as arc commonly en¬ 
countered. 

Sometimes during very calm weather the negative 
gradient formed during the da\time is not com¬ 
pletely destroyed at night. Then the next day’s sur¬ 
face heating is supcrimjioSed on the residual gradient 
from the previous day. Figure lb shows an example 


RESIRICTED 





















































DIURNAL AND SEASONAL CHANGES 


35 



I'k.i'rf, I I. Ratliylliermof^ranis illustrating diurnal Aranniiig. 



EiGi'Rr. 15. Bathytliermograins illu.strating diurnal wanning. 



uOCAL TIME 0615 0800 1000 1200 1407 1600 1800 0610 

WIND FORCE 33222 2 3 3 

FitiURE If). Bath\thcrinograins illustrttting diurnal wanning. 



WIND FORCE 4333343333 

I'lutiRK 17. Hathvthennograins illustrating diurnal wanning. 



































































THE BASIC VERTICAL THERMAL STRUCTURE OF THE OCEANS 




ot the very strong temperature gradients that can 
result from such a situation. Vertical mixing is re¬ 
duced to a minimum, parth because of calm weather, 
and also because turbidence decreases with increas¬ 
ing stability. I’hese temperature cur\es therefore aj)- 
proximate very closely the theoretical curve for the 
penetration of solar radiation. 

By contrast. Figure 17 shows a series of bathyther¬ 
mograms taken on a day when there was a gentle to 
moderate breeze which maintained a shallow mixed 
layer separated from the main l)ody of isothermal 
water below l^y a small thennocline. Whth still 
stronger winds the mixed layer would extend deeper, 
and generally when the wind force is 4 (Beaidort) or 
more, the downward distribution of heat takes place 
rapidly enough to prevent the formation of shallow 
negative gradients. 

Small amounts of diurnal warming occur fre- 
cpiently with little or no effect on sonar performance. 
However, the establishment of negative gradients at 
or slightly below projector level can cause marked 
reduction of echo ranges. The amount that the range 
is reduced depends not only on the total decrease in 


LOCAL TIME 






257 OBSERVATIONS 

Figure 18. Diurnal frequency of reduced ranges in vari¬ 
ous areas. 


tenijK'raturc in the water abo\e target depth, but also 
on the form of the temperature-depth curve. In gen¬ 
eral, however, a temperature difference ol 0.1 F or 
more between the surface and .SO feet will seriously 
reduce the range, and this has been chosen as a defini¬ 
tive \ alue in the Navy manuals. 

From the standpoint of subsurface warfare it is 
important to know under what conditions range re¬ 
duction can be expected. Since diurnal warming is 
most jjronounced in the afternoon, it is expected 
that range reduction will be most freipient at that 
time. Fhis is illustrated in Figure 18, which shows the 
jiercentage of jjeriscope depth ranges less than 1,500 
yards determined by the prediction rules used for the 
Sonar Charts. According to these rules, a temperature 
decrease of about 0.4 Fahrenheit in the top 30 feet of 
the ocean reduces the periscope-depth range to less 
than 1,500 yards. 4'o compute the data reejuired for 
Figure 18, all the available bathythermograms have 
been used for eight oceanic areas, whose positions are 
charted in Figure 19. The maximum frequency of 
short ranges occurs at about 1600 local time in all 
the areas, and the minimum frecpiency is at night. 
4'here are also geographic variations in the fre- 
([uency distributions, short ranges being more com¬ 
mon in coastal waters than in deep oceanic areas in 
the same latitude, and more common in high and 
mid-latitudes than in low. The frequency of range 
reduction is also greater in summer than in winter, 
and the difference is more pronounced in the higher 
latitudes, where there is a well-defined seasonal 
climatic cycle, than in the tropics where the weather 
is more uniform throughout the year. 

A complete analysis of diurnal heating would re¬ 
quire consideration of all the processes of heating 
and cooling at the sea surface. 1 he rariability of 
these processes depends on local weather and climatic 



KTSTiuCFlir 

- - -- 











































DIURNAL AND SEASONAL CHANGES 


37 


TEMPERATURE DECREASE 0 TO 30 FEET 
0° .S'* 1.0® 1.5® 2.0® 2.5® 3.0® 3.5® 



factors; hence these factors can be used to predict 
dinrnai heating'. The variahfes which appear to be 
most significant are the noon altitude of the sun, 
wind force, the degree of cloud coverage, the differ¬ 
ence in temperature between air and water, and the 
humidity of the air. 

In the spring of 1942 a 25-day study- of solar heat¬ 
ing was made in the Gulf of Mexico. Hourly BT 
lowerings were made during all this period and de¬ 
tailed meteorological records were kept. The prin¬ 
cipal conclusions drawn from these data can be sum¬ 
marized as follow's: 

1. During spring in the Gidf of Mexico the ellect 
of the wind on the downward penetration of heat can 
be plotted as in Figure 20. 14iis diagram shows the 
negative temperature difference to be expected as the 
wind strength and the heat balance vary. 

2. .-\t the time these observations were being car¬ 
ried out, a negative gradient of 0..S degree was con¬ 
sidered critical. To produce such a gradient with a 
force 4 wind the excess incoming heat, between the 
hours of 0800 and 1100, must exceed 200 calories per 
square centimeter, \\4th a force 3 wind the minimum 
heat A alue is 140 and with a force 2 wind it is 100. 

3. On no day was the residual heat sufficient to 
produce the critical temperature gradient with a 
force 5 wind. 

4. 1 he heat balance can be calculated from hourly 
meteorological obser\ations made during the morn¬ 
ing, or it can be estimated with somewhat less accu- 
rac)' from such a nomogram as shown in Figure 21, 


using the average cloud coverage and the average 
difference in temperature between the air and the 
water during the morning. 

5. On only two days did the morning observations 
fail to forecast the afternoon temperature gradients 
as well as coidd be known by computing the heat 
balance for the whole day. On these exceptional days 



10 

9“ 

8° 

7“ 

6“ 

5“ 

40 

3“ 
20 
I ° 
0 ° 
- 1 ° 
-2 
-3 


o 


o 


o 


LlI 

a: 

3 

h- 

< 

tr 

UJ 

CL 

S 

UJ 


2 

< 

(/) 


UJ 

a: 

3 

< 

cr 


UJ 

I- 

cc 

UJ 


FicaiRE 21. Nomogram for delermiiiing heat Ijalance in 
the Gulf of Mexico. 


RTSTRJCriDl 















38 


THE BASIC VERTICAL THERMAL STRUCTURE OE THE OCEANS 



Eiru'RF. 22. Tcni])eiatiuc'-de]>th curves from a Pacific 
weather station illustrating tiiurnal warming. 


overcast skies in the morning changed to clear in the 
afternoon. 

Similar series of bathythermograms have been 
taken at a weather patrol station in the northeastern 
Pacific (4()'’N, 150°\\^). They serve to show several 
additional characteristics of diurnal warming which 
shotild be included in the present discussion. Twen¬ 
ty-four daily series of bathythermograms were ana¬ 
lyzed. Each series consisted of observations covering 
the period from 0600 to 2000 local time, plus at least 
one from the following morning. In each case the first 
observations showed either completely isothermal 
water or isothermal water underlying a slight posi¬ 
tive gradient. Ihen there developed at the surface a 


negative gradietit which sometimes reached a depth 
of nearly 50 feet. By the following morning this had 
completely disapjjeared and the water was once more 
isothermal or slightly cooler at the surface. For each 
series the temperature was measured at 5-ioot depth 
intervals and plotted in Figure 22 as the average dif¬ 
ference between the temperature at each depth and 
the temperature at 50 feet. 'Fluis an average picture 
is obtained of the amount of heating and cooling 
over a period of several days. 

Figure 23 presents the same measurements in a 
different way. Here the a\erage increase in tempera¬ 
ture above that at the 50-foot depth is plotted against 
time, for each 5-foot level. This makes it possiljle to 
compare easily the amount and rate of temperature 
increase at different depths. The figure shows that the 
greater the depth, the later the hour when the water 
reaches its maximum temperature. This is because at 
anv considerable distance below the surface the uain 
in heat is not so much from absorption of radiant 
energy as from downward conduction by vertical 
eddy motion, which takes place increasingly slowly 
at greater depths and causes a prf)gre.ssi\’e lag in the 
rate of heating. 

llius far the discussion has largely centered 
around the simplest cases of diurnal warming, in 
which the negative gradient represents the effects of 
1 day’s heating. From the jnactical standpoint, how¬ 
ever, it is important to consider all cases of surface 


DIURNAL TEMPERATURE INCREASE 


oooopp — 

ro o fo ’-ti OS 00 o 



FuaiRE 23. Diurnal temperature change at various depths from the surface to 50 feet. 


RESTRICTED 


















39 


DIURNAL AND SEASONAL CHANGES 



Figure 24. Relationsliip between .shallow teinperatiire 
gradients and wind at difFercnt seasons. 


negative gradients, both the simple ones and those in 
which a period of calm weather has permitted the 
accumulation of a negati\ c gradient for several days. 
For this purpose a large group of BT observations 
from the Pacific have been examined from the stand¬ 
point of the weather conditions at the time the ob¬ 
servations were made. 

In Figure 24, the upper graph shows the frequency 
of negati\’e gradients for different wind forces for 
each season of the year. Any measurable decrease in 
temperature between the surface and a depth of 30 
feet was called a negative gradient. In the lower 
graph are plotted the median values of those tem¬ 
perature differences which were not zero; if the i.so- 
thermal cases had been incltided, a majority of the 
median calues would, of course, have been zero. 
Thus, the two parts of Figtire 24 must be considered 
together: the upper part shows how often measurable 
negative gradients occur; the lower, their relative 
strength when they do occtir. In general. Figure 24 
shows the greater frequency of negative gradients 




Figure 25. Relationship between shallow temperature 
gradients and wind at dilterent latitudes during the sum¬ 
mer season t june, Jnly, and .\ugust). 


during the warmer part of the year, and it also sug¬ 
gests that seasonal changes affect the magnitude of 
the gradients more than they alfect the frequency of 
their occurrence. A possible weakness of the figtire is 
that it combines data from a wide range of latitude. 
Perhaps most of the winter observations showing 
negative gradients came from relatively low latitudes. 
It is to be hoped that as bathythermograms become 
more numerous the various factors influencing the 
formation and persistence of tenq^erature gradients 
may be more clearly separated. 

Figure 25 shows the effect of wind on surface heat¬ 
ing at different latitudes during the summer season. 
In the wintertime the difference between high lati¬ 
tudes and low latitudes is undoubtedly much less, if 
not actually reversed. 

Both Figures 24 and 25 are in general agreement 
with pre\'ious statements that tv'ind-generated ver¬ 
tical turbulence is destrtictive to stn face temperature 
gradients. But whereas in the previous disctission it 
was stated that diurnal heating does not develop 


RESTRICTED 
















^0 


THE BASIC VERTICAL THERMAL STRE CTl RE OF THE OCEANS 




Figire 26. Relationship bettveen shallow temperature 
Gradients and cloud coveraaie. 


Figire 27. Relationship between shallow temperature 
gradients and cloud coverage. 


when the tvind is much more than force 3 (Beaufort), 
here it is apparent that a small but significant num¬ 
ber of negati\e gradients are found when the wind 
force is 5, 6, or even 7. The difference lies in the fact 
that any average set of observations will contain a 
number of cases in which a strong negative gradient 
is built up during se\ eral days of calm ^veather. Then 
the stability so obtained requires a much stronger 
wind to destroy the gradient than tcoidd have been 
needed to prevent its formation in the first place. 

The Pacific observations have also been correlated 
with cloud coverage. The usual method of evaluating 
clouds by estimating the per cent of sky covered is by 
no means an accurate measurement of the extent to 
which clouds reduce solar radiation. Nevertheless, 
Figures 26 and 27 show a significant relationship be¬ 
tween the amount of clouds and suppression of sur¬ 
face heating except in the morning obser\ations. In 
this case it seems likely that during the hours of the 
day when back radiation exceeds insolation, clouds 
may sometimes serve as a blanket holding in the sur¬ 
face water’s heat. 

The effect on diurnal warming of the difference 
between air and sea temperatures has not yet been 
analyzed thoroughly except in the small group of 
Gulf of Mexico observations previously referred to. 
There it proved to be one of the more important fac¬ 
tors. Morecjver, the available data on the geograph¬ 
ical distribution of air-sea temperature difference 
(Figure 28) indicate that areas where this difference is 
largest coincide with regions of pronounced diurnal 
warming. 

The whole problem of surface heating is com¬ 
plicated by the fact that the various processes are so 


closely interrelated. The quantitatise relationships 
cannot be solved without a great deal more scientific 
work on the processes involved and statistical exami¬ 
nation of the observations. Flowever, it is not too 
much to expect that eventuallv echo ranges will be 
predicted in much the same way that weather fore¬ 
casts are made. 

CO.WECTIOX 

Convection has been mentioned frequently in con¬ 
nection with heat exchange at the surface. It has been 




-X / /Jv\\ //i^^ //^ 

\\//7l\ \\//7I(\ \\// 4 

\//A\\/ 


\ // \ \ // 4 









Figure 29. Diagrammatic representation of convection 
cells. 


RESTKICTTI) 






























1 



Figure 28. Chart showing the difference between water and air temperatures in the Northern Hemisphere, summer (above) and winter (below). 



































































































































































































A 




r 




( 




I 









»'i 


^<■1 





1 




» 


i 

» 




I 


* 


• ' I 





M i 

% 

t 

, i -. 

4 






^! .■ 


A 

f 



f 





‘» j 











DIURNAL AND SEASONAL CHANGES 


41 


apparent that much of the time the sea surface is 
being cooled by evaporation, back radiation or con¬ 
duction. Thus a very thin layer of water develops at 
the surface, which is cooler and therefore denser than 
the water just below, jnoducing an unstable situa¬ 
tion and causing the cool water to drain downward 
through the warmer water below. This convection 
process is generally accompanied by movement due 
to wind friction, and the effects of the two are not 
easily separable. 

Studies of convective motion in the surface layer‘d 
were begun only a year or two before the war. Pat¬ 
terned motions in the surface of a lake were reported, 
and attention was drawn to the fact that floating gulf 
weed in the North Atlantic was sometimes distrib¬ 
uted in lines extending up and downwind and some¬ 
times scattered at random. It is strange that the long 
lines of gulf weed which are so common in the 
warmer areas of the North Atlantic had never before 
been connected with a patterned flow, involving al¬ 
ternating bands of convergence and divergence of the 
surface water. This can be pictured diagrammatically 
as in Figure 29. It follows then that when gnlf weed 
is sufficiently plentiful so that some of it gets caught 
up in each convergence, an estimate of the thickness 
of the almost isothermal layer can be easily made. If 
the convection cells were circular in cross section, the 
crosswind spacing of the weed would be just twice the 
depth of the mixed layer, .\ctually the observations 
which are at hand indicate that the average ratio is 
nearer 1.8 and thus in Figure 29 the cells have been 
shown as being slightly compressed laterally. 

Experiments with drifting bottles and observations 
of the orientation in relation to the wind of the float¬ 
ing jelly fish, Physalia, indicate'* that the effect of the 
earth’s rotation is to increase the vigor and width of 
the cells rotating to the right (in the Northern Hemi¬ 
sphere), so that the circulations on the average have 
the form shown in Figure 30. 

Obviously the wind controls the orientation of 
these cells and contributes some of the energy re¬ 
quired to maintain them. However, surface cooling is 
a critical part of the process. \\’hen the air is warmer 
than the water and when evaporation is low, well- 
develojjed convection cells are not encountered. How 
much surface cooling is required to initiate them is 
not known. 

It is probable that at low wind velocities convec¬ 
tion may take quite a different form, for it is a com¬ 
mon sight in calm weather to find the gulf weed 



rafted together in roughly circular masses, indicating 
that the cooled surface water may be draining down¬ 
ward at scattered points in much the same manner 
that “chimneys” of rising air develop at low wind 
velocity when the air is being warmed from below. It 
also seems possible that beyond some critical maxi¬ 
mum wind velocity, which probably varies with the 
rate of surface cooling, the patterned type of convec- 
ti\e flow breaks do^vn and the motions become ran¬ 
dom. But however this may be, it is clear that over a 
fairly wide range of wind velocities patterned con¬ 
vection plays an important part in maintaining a 
surface layer which is nearly isothermal vertically. 
I’he convecti\'e layer may be shallow and as a whole 
gaining in heat, as when diurnal warming is acti\e, 
but as long as the surface itself is being cooled some 
form of overturn will be maintained. 

If this general idea of convective motion is ac¬ 
cepted, then for present purposes several general 
conclusions can be drawn, although it is admitted 
that these matters are not as yet on very firm ground: 

1. Convection will be most active where the hu¬ 
midity is at a minimum and where the air tempera¬ 
tures are lowest in comparison to surface tempera¬ 
tures (see Figure 28). 

2. The more active the convection, the greater is 
the probability that a deep and virtually isothermal 
layer will be found at the surface; that is, tempera¬ 
ture conditions will be favorable for echo ranging. 

3. AVithin the convective layer, sheets of slightly 
colder, downward-mo\ing water can be expected. 
Under some conditions this may be responsible for 




RESTRICTED 


J 

















42 


THE BASIC VERTICAL THERMAL STRUCTURE OF THE OCEANS 



A, stable surface layer; 1), surface heating and formation of a second stable sur- 

H and C, stages in mixing. face layer; 

K, continued mixing and secondary thermocline. 

Figure 31. Diagram of rvind-stirring. 


the clevelopnieiit of “iiiicro.striictiire,” that i.s, small- 
scale vertical \ariations in temjreratiire anti salinity, 
which may have an unfavorable elfcet on echo rang¬ 
ing or at least make the strength of echoes highly 
N'ariable. Further study of these factors will be re¬ 
quired, however, before it will be possible to assess 
their j^ractical value properly. 

Effect of W'ind 

I'he amount of energy rct{uired to produce vertical 
mixing in the surlace layer depends upon the density 
gradient. If this is negative, that i.s, if the density is 
greatest on top, the water is unstable and already has 
the {potential energy retpiired for \ertical movement. 
If the density gradient is zero, in other words, if the 
water is of uniform density, a sufficient force must lie 
applied to overcome the frictional resistance of the 
water. Finally, if the density increases with depth, the 
force required to produce vertical mixing must be 
strong enough to overcome the difference in weight 
between layers ol water of different density, d’hus, 
water whose density gradient is positive is highly 
stable. Nevertheless, such water is fairly suscejrtible 
to lateral movements which follow along the planes 
of equal density. .Such horizontal motion entails fric¬ 
tion between two layers of dillcrent velocity, and 
gradually water from the lower layer is dragged into 
the iqjper and \ ice \ersa. I hus, in time a positive 
density gradient can be weakened and destroyed by 
wind-generated frictional forces which at first acted 
only laterally. 

In the absence of surface cooling, the wind, if it is 
sufficiently strong, is capable of forming or deepen¬ 


ing a mixed surface layer. In other words, the fric¬ 
tional effect of the wind is important, whether or not 
it is aided by convection. A thermally stable surface 
layer once formed will be broken down by a wind, 
j)ro\ided it blows hard enough and long enough. 
Especially in the latitudes of variable winds the ef¬ 
fects of wind-stirring can be clearly seen on many Bd’ 
records. In fact, with spring obser\ations it is some¬ 
times possible to interpret the record in terms of the 
winds during the past week or more. 

A hypothetical case is illustrated in Figure 31. The 
initially stable situation (Clurve A) was changed by 
a wind, first to Curve B and later to Curve C. There 
followed another period of warming and light winds 
so that Cur\e I) de\eloped. As the wind increased 
again this became Cur\ e E. d his is the explanation 
of much of the thermal variability at shallow depths. 

It is inq)ortant to notice in Figure 31 that as the 
mixed layer forms, the thermal stability just below 
increases. This same phenomenon will be evident 
during the autumn months in the diagrams showing 
the seasonal cycle. Figure 35 for example, ddte sharp¬ 
ness of the iq3j)er part of the thermocline increases as 
the depth of the mixed layer increases as a result of 
local cooling and wind-stirring, and this is of im¬ 
portance in considering layer effect. 

d he effect of the wind in maintaining or deepen¬ 
ing an isothermal surface layer is comj^licated by the 
fact that the wind also sets iqr a current near the sur¬ 
face which may remove the mixed water from a given 
area almost as fast as it is formed. Presumal)ly such a 
current requires some time to develop (see Section 
y.l.^^l), while the eflect on vertical mixing is more im- 


RE.SFRICFEl) 
























































































































































DIURNAL AND SEASONAL CHANGES 


43 


mediate. Thus, brief and variable winds are nsnally 
responsible for the elfecis illustrated in Figure 31. 

Waves and Wave Transport 

W'aves are among the most ob\ ions of marine phe¬ 
nomena, and they have always had a direct and no¬ 
ticeable eHect upon shi}is, as any ocean traveler 
knows. Antisubmarine ships are no exception, and 
esj^ecially in the case of smaller vessels, the motion 
produced by the waves may be a serious deterrent to 
efficient echo ranging. In addition, the process of 
echo ranging itself is affected and sometimes mark¬ 
edly impaired by the noise of breaking waves and by 
cpienching effects. 

.\side from their direct effects on echo ranging, 
wa\es play an important indirect role through their 
influence on local oceanographic conditions. To un¬ 
derstand these effects it is necessary to examine some 
of the simple properties of wave motion. 

In its simplest form a wave is a regular, progressive 
displacement of the surface from its mean level. How¬ 
ever, the actual appearance of the open sea seldom 
gives any impression of rhythmic regularity. The 
j)icture is frequently one of apparently random move¬ 
ment and chaotic disorder. Elevations and depres¬ 
sions of \arious shapes and sizes appear and disap- 
jjear; some are superimposed iqjon others; they may 
travel at different speeds and even in different gen¬ 
eral directions. Nevertheless, the deformation of the 
surface at any given place and time may, with some 
allowance for turbulence, be thought of as the com¬ 
bined result of some finite number of relatively 
simple wave trains of varying periods, amplitudes, 
velocities, and crest lengths. 

simj)le wave can be completely described by 
three or, at the most, four measurements. First there 
are the wave length, which is the distance from crest 
to crest or from trough to trough, and the wave 
period, which is the time in seconds required for the 
jDassage of one wave length. I’he ratio of these two 
cjuantities gives, of course, the speed of the wave. 
Fhen there is the height of the wave from trough to 
crest. Finally there is the crest length. For most waves, 
the crests (and troughs) are assumed to be of indefi¬ 
nite length. T here exist, however, both in theory and 
in fact, wax es xvhose crests are broken up into regtdar 
segments separated by segments of troughs. To de- 
scribe such xvaves fully one must specify the length of 
these crest segments. This is the crest length and such 
wax es are called short crested xvaves. 


Beloxv a certain xery loxv speed the xvind can bloxv 
xvithout causing so much as a ripple. Once a rising 
xvind has begun to form xvaves, it xvill increase their 
size and speed rapidly by a combination of pressure 
on the xvindxvard slopes and suction on the lee. The 
speed of xvaxes so propelled is obviously limited by 
the speed of the xvind, and observations indicate that 
generally the largest xvaves have a velocity about 
eight-tenths that of the xvind. 

The limit to a xvave’s steejjness is relatively loxv. 
Unless the length of a xvaxe is at least 7 times its 
height, the xvaxe xvill break. Waves may reach the 
breaking point either through the action of the xvind 
or through reinforcement by some other xvave train 
of different period. AVhenexer xvaxes break, or even 
approach breaking, the simple xvaxe form is dis¬ 
torted and there is a marked degree of turbulence. A 
xvind of force 5 (Beaufort) or more augments this 
mixing effect by bloxving the xvave crests across the 
trough ahead in the form of spume. 

With a rising sea, xvaves of various lengths are 
present. The smallest xvaves break at loxv xvind veloci¬ 
ties, xvhile the longer ones remain and attain a greater 
height, their size being finally limited by the xelocity 
and duration of the xvind and the distance traveled. 
The tendency of xvaxes to increase in length xvith the 
distance traveled and for their speed to increase cor¬ 
respondingly may occasionally lead to the surprising 
result that the sxvell folloxving a storm has a higher 
velocity than the xvind xvhich originally produced it. 

If an observer at sea xvatches a gull or other object 
floating on the xvater, he xvill see that as a xvave 
passes, the movement of the object is not merely up 
and doxvn but rather it describes a roughly circular 
or elliptical orbit, in such a manner that at the crest 
the object moxes forward xvith the wax^e and in the 
trough it is moving in the opposite direction. Par¬ 
ticles of water in the xvave moxe in somewhat similar 
orbits. Furthermore, the movement occurs not only 
in particles of surface xvater but also in those some 
distance beloxv the surface. T he orbits of movement 
are bigger for large xvaves than for small, but for any 
xvave the size of the orbit and the velocity decrease 
rapidly xvith depth, ajjproaching zero at a depth of 
more than half a xvave length beloxv the mean surface 
level. 

The fact that the velocity decreases continuously 
xvith depth prevents the orbits from being completely 
closed circles. That is, a xvater particle traxels faster 
in the iq:)per part of its orbit than in the loxver, and 




RESTRICTED 








44 


THE BASIC VERTICAL THERMAL STRUCTURE OF THE OCEANS 



Q 13 17 26 9 13 26 II 28 18 31 17 12 3 30 17 27 25 19 


MAR APRIL MAY JUNE JULY AUG. SEPT OCT NOV DEC JAN FEB 

Figure 32. Typical liathvthermograms showing sea.sonal temperature cycle in the Bermuda area. 


SO after having completed one revolution does not 
return to the point from which it started but to a 
point slightly farther forward. Thus there is a trans¬ 
port of water in the direction of the waves’ progress. 

The rate of this forward transport is greatest at the 
surface and decreases rapidly with depth. The total 
\olume transport is dependent on the wave’s length 
and height. \\’ith a very low wave in deep water it is 
negligibly small. AVith larger waves the transport is 
probably of the same order of magnitude as wind 
driven currents. 

In summarizing the effects of waves on the tem¬ 
perature structure of the ocean it is important to 
compare the direct effects of wind and the indirect 
effects resulting from wave motion. In each case there 
is mass transport of water, caused on the one hand 
by the frictional drag of the wind, and on the other 
by the character of the movement of water particles 
in the wave. Both are therefore capable, by transport¬ 
ing surface water, of producing zones of convergence 
or divergence, accompanied by raising or lowering of 
the thermocline, which alters di\ ing and sound con¬ 
ditions locally. Or they may be responsible in some 
cases for water of one temperature being carried over 
water of another. If the water on top is cooler than 
that below, marked convection will result. If it is 
warmer there will be stable stratification. 

Both wind and waves are also capable of altering 
sound conditions by causing vertical turbulence. 
Mixing by the wind results both from convection and 
from frictional drag caused by the horizontal trans¬ 
port of water. Mixing by waves occurs chiefly when 
they break; there is no evidence to indicate that wave 
transport produces a measurable amount of tur¬ 
bulence. 

Thus wind and waves have very similar effects on 
oceanographic conditions and echo ranging, al¬ 
though the effects are produced in tjuite different 
ways. Because they are so similar, the effects are not 


easily separable, and quantitative investigations have 
only just begun. 

5.2.2 Yhe Seasonal Thermocline 

General DESCRiPxtoN 

AVe have seen that, except for the short-lived nega- 
ti\e temperature gratlients of diurnal warming, con¬ 
vection and wind-mixing usually maintain a surface 
layer of substantially isothermal water. However, 
during the time of year that insolation markedly ex¬ 
ceeds heat losses, this mixed layer seldom extends 
down to the main thermocline. This is partly because 
convection is efficient only to the depth that the water 
is unstable, or at least of uniform density, and partly 
because during that time of year winds are ordinarily 
mild. In any case, as the surface water gradually 
grows warmer with the advance of spring, there de¬ 
velops, immediately below the depth to which con¬ 
vection and wind-mixing remain effective, a second¬ 
ary thermocline. If the tenqjerature of the layer 
above this thermocline continues to rise, the thermo¬ 
cline itself will grow steeper and become more of a 
barrier to downward mixing. 

A typical seasonal thermal cycle in mid-latitudes is 
shown in Figure 32 by a series of bathythermograms 
taken in the Bermuda area and in more diagram¬ 
matic form in Figure 33. In midwinter the water 
column is essentially isothermal from the surface to 
the top of the main thermocline (Figure 33A). Start¬ 
ing in March the surface water begins to warm, and 
the depth of wind stirring is at once markedly de¬ 
creased (Curve B, Figure 33). Thus in spring the 
water column from top to bottom consists of the fol¬ 
lowing parts: (1) a relatively shallow, wind-stirred 
surface layer which is rajjidly increasing in tempera¬ 
ture, (2) a relatively shallow layer of transition, the 
seasonal thermocline, (3) an isothermal layer which 
retains midwinter temperature, (4) the main thermo- 


RESTRICTED 







































DIURNAL AND SEASONAL CHANGES 


45 


dine, and (5) the deep water. Until midwinter again 
these five subdivisions can be made out in any tem¬ 
perature-depth curve in mid-latitudes. In high lati¬ 
tudes there is, of course, no permanent thermocline, 
and in the tropics because of the small seasonal 
change it is difficult to distinguish between the per¬ 
manent and seasonal thermocline. By midsummer 
over the greater part of the oceans, surface warming 
and vertical turbidence have increased both the 
depth and the magnitude of the seasonal thermocline 
(Curve C, Figure 33). During the autumn (Curve I), 
Figure 33) cooling at the surface and the resulting 
tendency for instability rapidly increase the thickness 
of the wind-stirred surface layer. The seasonal ther¬ 
mocline also deepens rapidly, but at the same time it 
decreases in magnitude, only to vanish as the simple 
midwinter conditions are restored. 

By comparing Figure 33 and Figure 14 it will be 
seen that the seasonal cycle bears a marked resem¬ 
blance to the diurnal cycle, if the change in scale be 
neglected. Thus the curves for winter and night, for 
spring and morning, for summer and afternoon, and 
for autumn and evening are each of very similar 
shapes. 

Obviously, seasonal changes are at a minimum 
near the equator. Furthermore, the changes do not 
come simultaneously over the whole ocean basin, but 
work gradually northward and southward with the 
sun. An additional complicating factor is that the 
winds are not steady outside the trade-wind belt so 
that the development and the decay of the seasonal 
thermocline do not always proceed steadily. Never¬ 
theless, once the seasonal thermocline has been well 
established (by May, for example, in mid-latitudes) it 
w'ill not disappear, no matter how great or how' many 
the storms, until late in the autumn. This funda¬ 
mental quality of a thermocline, namely, that it has 
stability and can thus resist vertical turbulence, 
should be borne in mind throughout the reading of 
this report. 

Geographical Factors 

The main (or permanent) thermocline is respon¬ 
sible for the vertical temperature gradients encoun¬ 
tered by a deep stdmiarine over only a rather small 
fraction of the total ocean area, chiefly in the tropics. 
The influence of the seasonal thermocline, on the 
other hand, is very widesj^read. 

This leads to an analogy which can be drawn be¬ 
tween land warfare and subsurface warfare, and 



FicaiRK 33. Diagram of seasonal thermocline in the Ber- 
mnda area, .\.^\'inter. B.,Spring. C.Summer. D..\utumn. 


which may serve at this point in the discussion to 
clarify somewhat the roles of the various types of 
thermoclines. In land lighting, as is well known, sol¬ 
diers are very conscious of two factors: the weather 
and the terrain. In subsurface warfare the weather 
also plays an important part in that by controlling 
diurnal warming it determines the temperature gra¬ 
dients at and near projector level. These in turn de¬ 
termine the refraction pattern and thus the general 
sound conditions. But there is also in the sea a 
physical structure analogous to terrain in land war¬ 
fare. The sea,sonal thermocline, because of layer 
effect, is comparable to a hill behind which a sub¬ 
marine can hide. At times and at places where the 
seasonal thermocline is sharp and well developed, 
liy submerging to below the isothermal surface layer 
a submarine may be able to gain very considerable 
cover. Other parts of the sea, from this standpoint, 
because of the absence of vertical thermal gradients, 
are comparable to a level plain in land warfare. This 
is a rather new' and far-reaching conception. It means 
that both the submarine and her adversaries wall 
find it an advantage to know the subsurface tempera¬ 
ture distribution much as a soldier has to know' the 
topography, both for local cover and for strategic 
planning. 

One can gain a good picture of the magnitude and 
geographical variations of the seasonal thermocline 
merely by subtracting midwinter surface tempera- 
ttires from midsummer surface temperatures. The 


RES! RICTED 














46 


THE BASIC VERTICAL THERMAL STRUCTURE OF THE OCEANS 


68.5° 67.1° 67.2° 69.0° 72.2° 75.9® 79.5° 80.6® 80j4 ® 77.9° 74.8® 70.8® 



resiiliing chart, Figure 34, shows, as would be ex¬ 
pected, that tlie greatest seasonal change occurs in 
mid-latitudes, especially in the western parts of the 
oceans where the prevailing winds are off the con¬ 
tinents. It will also be noticed that in those areas in 
the eejuatorial regions where the seasonal thermo- 
cline is less than 2.5 degrees the main thermocline is 
largely above the depth of 600 feet (see Figure 8). In 
these areas, therefore, the main thermocline rather 
than the seasonal is of major importance in determin¬ 
ing layer depth and there is comparatively little 
change throughout the year. 

The fact that the seasonal thermocline so domi¬ 
nates the vertical temperature distriljution at depths 


critical for a modern sid)marine greatly increases the 
difficidty of getting out practical charts of either the 
sound or the diving conditions. Since seasonal 
changes are continuous, though more rapid in spring 
and autumn, it is sometimes misleading to group 
observations for periods e\'en as short as a month. 

Fhe Seasonal Progression 

I’here are two Cjuite different ways in which the 
available data can be plotted to demonstrate for our 
present purposes the significant points in connection 
with the seasonal temperature cycle. It is possible to 
show average monthly temperature-tlepth curves 
(Figure 35, for example), or it is possible to plot iso- 



RTSTlUCri^ 










































































































































































































































































DIURNAL AND SEASONAL CHANGES 


47 



Figure 37. \’ai'iabiliiy in individual hatliytliermo«ram.s 
in tlie Beiiniula area in April. 


therms with depth and time as coordinates (Figure 
36). Both methods have certain advantages and dis¬ 
advantages. 

d'he ligtires show the available data from near 
Bermtida and are typical of the seasonal changes over 
wide areas of the warmer waters of the western North 
.Atlantic. I'he temperatnre-depth curves (Figure 35) 
show the average of the available BT data and there¬ 
fore indicate the type of conditions most likely to he 
encountered. Some variation in individual bathy- 
thermograms is to be expected. To give some idea of 
the degree of variability in the Bermuda area a se¬ 
lection of the observations for .April has been plotted 
in Figure 37. 

It will be noted in Figure 35 that Irom May 
through .August on an average the warm surlace layer 
is thermally stable (surface temperature minus the 
temperature at 30 feet is greater than 0.3 F) to a 
degree which would limit the maximum range on a 
shallow target. This tendency for negative gradients 
near the surface in the summer reflects the rather 
moderate winds near Bermuda at this partictdar sea¬ 
son, plus the rapid warming due to clear skies. FTr 


example, of the 360 bathythermograms available 
from May to .August in this area, 65 per cent show an 
isothermal surface layer varying in depth between 
40 and 100 feet, while 35 per cent show thermal sta¬ 
bility at and near projector level. The average curve 
then will he one having weak negative temperature 
gradients near the surface, and this is somewhat mis¬ 
leading. The same applies to a lesser degree to 
September. 

If the oh-servations were more numerous, only 
Ijathythermograms taken at night or during periods 
of strong winds might be used to develop average 
temj)erature-depth curves and in this way the effects 
of diurnal warming could he eliminated in such a 
diagram as Figure 35. However, as yet there are too 
few observations from most months to make this de¬ 
sirable. On the other hand, in Figure 36, since iso¬ 
therms for only every one degree are used, the effects 
of diurnal warming are eliminated. Nevertheless, 
from this type of diagram it is somewhat more difh- 
cidt to judge the refraction pattern. 

Figures 38 and 39 show the same kind of gra})hs for 
the seasonal cycle in mid-latitudes in the Pacific.** 
The differences between these two temperature cycles 
are very slight. "Fhe depth of the winter mixed layer 
appears to be slightly greater in the Atlantic than in 
the Pacific, and the development of the seasonal ther- 
mocline in spring is perhaps less regular. Both these 
differences are due at least in part to the fact that in 
the area of the .Atlantic chosen for analysis the fre¬ 
quency of winds of gale force is 10 to 20 per cent 
greater during the winter and spring months than in 
the Pacific area. It is reasonable to suppose that when 
observations from more regions have been analyzed. 


a Figure 38, like Figure 35, was prepared by plotting the aver¬ 
age temperature dillerences from tbe surface to certain fixed 
depths and then making the main break in each curve occur at 
the average layer depth. Fhe bathythermograms for Figure 38 
were read at fewer depths than those for Figtire 35, and so no 
attempt has been made to join the points by a smooth curve. 



Fu;urf. 38. .Seasonal cvcie in mid-latitudes in the Pacific—average monthly temperature-depth curves. 


j^RE.S'I RIC I ED 



























































































































































DEPTH IN FEET DEPTH IN FEET DEPTH IN FEET 


-48 


THE BASIC VERTICAL THERMAL STRUCTURE OF THE OCEANS 



Figure 39. Seasonal cycle in mid-latitndes in the Pacific—average isotherms. 



Figure 40. Seasonal cycle in the ecjnatorial Pacific—average isotherms. 



Figure 41. Seasonal cycle over the continental slope of the western North Atlantic—average monthly temperature- 
depth curves. - - 


RE.STRTCTED 





















































































DIURNAL AND SEASONAL CHANGES 


49 



many such minor diflerences in seasonal cycles will 
be found, which can be correlated with local climatic 
\ ariability. 

In the trade wind belt the magnitude of the tem¬ 
perature change between winter and summer be¬ 
comes less pronounced than it is in mid-latitudes, but 
the pattern of the change is in other respects very 
similar. The annual temperature change becomes 
least near the equator and may be no more than 4 or 
5 degrees, as shown in Figure 10. Here the form of the 
seasonal cycle is t[uite different from those previously 


described, for there is a bimodal seasonal curve, with 
the highest temperatures occurring in April and in 
October slightly after the time when the sun has 
jjassed directly overhead. 

Going to the opposite extreme, very marked sea¬ 
sonal cycles are found in the higher latitudes not far 
from land. Fhis is shown in Figures 41 and 42, in 
which obser\’ations from between the northern edge 
of the Gidf Stream and the continental slope in the 
sector off southern New England have been plotted 
by the same methods previously used. Here the maxi- 



RESTRICTED 












































































50 


THE BASIC VERTICAL THERMAL STRUCTURE OF THE OCEANS 


niiiin temperature difierenee Ijetween summer and 
winter is ol the order ol 2() degrees as compared with 
about hall as much as is shown in Figures 35 and 3(). 

In high latitudes and at greater distances Irom the 
continents the seasonal changes become less pro¬ 
nounced again; this rellects the smaller range in sur¬ 
face temperature (see Figure 31) and the stronger 
winds. Unfortunately the available observations are 
too few from any one area in high latitudes to demon¬ 
strate the complete annual cycle. The best data are 
from the Aleutian Islands area (Figure 43) and, as 


would be expected, emphasize the shoiler summer 
season. 

It is hoped that this small sample (Figures 35-13) 
will have served to illustrate the dc\elo])ment and 
the decline of the seasonal thermoclinc in the open 
ocean. It would reepdre a great many such diagrams 
to illustrate the slight differences encountered from 
region to region. W'hcn the number of B F observa¬ 
tions becomes sufficient to permit this to be done in 
considerable detail it will be possible to improve the 
charts of average echo ranging conditions. 




RESTRICTED 









Chapter 6 

RELATIONSHIP OF SALINITY AND TEMPERATURE 


6 1 TEMPERATURE-SALINITY 

CORRELATION IN THE DEEP WATER 
AND IN THE MAIN THERMOCLINE 

I N THE surface waters the relationship between tem¬ 
perature and salinity is highly \'ariable since it is 
affected by surface heating and cooling, evaporation, 
and precipitation. In the main thermocline and the 
deep water layer, however, the temperature-salinity 
correlation remains nearly constant within large 
areas. Knowing the temperature in situ, one can pre¬ 
dict the salinity of a sample of water with almost as 
much accuracy as it is measured by the routine 
jji'ocess (±0.02 "/()()). This is illustrated by Figure 1 
in which the observations ha\ing temperatures 
colder than 66 F from nearly all the available ocean- 
ograj)hic stations in the central part of the North 
Atlantic have been used. But when the water masses 
from widely different areas are compared, slight dif¬ 
ferences in the temperature-salinity correlation can 
be distinguished. For example, the bottom water 
originating in high southern latitudes of the Atlan¬ 
tic has a salinity of about 34.8 as compared with 
about 34.9 in the North Atlantic. Such small but 
persistent differences provide oceanographers with 
an excellent method of tracing the slow water move¬ 

SALINITY %o 



ments in the main thermocline and below. However, 
for our present purposes it is to be concluded that 
as long as a stdjinarine is operating in the main 
thermocline, the density can be predicted with very 
satisfactory accuracy merely by observation of the 
temperature. 

It will also be seen from Figure 1 that as the tem¬ 
perature increases, the salinity also increases. It fol¬ 
lows then that within the main thermocline salinity 
tlecreases with depth. This is the general rule in the 
open ocean and, as will be discussed in more detail 
below, is in marked contrast to the conditions in 
coastal waters where nearly always salinity increases 
with depth. 

From the point of view of the stability of the whole 
water column in the deej) ocean, the vertical distri¬ 
bution of salinity tends to decrease slightly the sta- 
bility due to temperature alone. In other words, if 
there were no decrease in salinity with depth, density 
woidd increase with depth slightly more rapidly. 
However, the vertical salinity variations are so minor 
that at a typical oceanic station at depths below the 
top of the main thermocline the observations for tem¬ 
perature and density fall along nearly parallel curves 
when suitable scales are chosen (Figure 2). It should 
be pointed out also that at depths where the vertical 


TEMPERATURE 



Figure 2 . Comparison between temperature-depth and 
density-depth curves in the mid-.\tlantic. 


RESTRICTED 


51 



















RELATIONSHIP OF SALINITY AND TEMPERATURE 


temperature gradient increases, the ^'ertical salinity 
gradient also increases. 

It will be seen from Figure 1 that in the central 
basin of the North Atlantic the waters at mid-depths 
are slightly fresher than woidd be the case if they 
were formed locally by the mixture of surface and 
bottom water. I'his situation also occurs in the other 
oceans near the axis of the main thermocline and is 
even slightly more pronounced. It is explained by the 
fact that when the stability is great, lateral mixing is 
more effective than vertical mixing. This is similar to 
the phenomenon described in connection with the 
establishment of temperature gradients, in which a 
force sufficient to overcome the frictional resistance 
of the water will cause mixing in water of uniform 
density, but a greater force is required to destroy den¬ 
sity differences. Therefore when there are density 
gradients, most mixing occurs laterally following the 
surfaces of constant density. Thus the temperature- 
salinity correlation at mid-depths can be explained 
by following the surfaces of constant density north or 
south until they intersect the sea surface. In short, 
they have the temperature and salinity character¬ 
istics they acquired at or near the surface in mid¬ 
winter in the higher latitudes, and in the course of 
their transport to the main thermocline region of the 
central basins they have been little modified by ver¬ 
tical mixing. 

62 SALINITY GRADIENTS NEAR THE 
SUREACE 

Salinity gradients arise (1) from the dilution of the 
surface by rainfall, melting ice, and the runoff from 
the land, (2) from evaporation of water from the sea’s 
surface, and (3) from the flow of waters of different 
salinity over one another as the residt of ocean 
currents. 

In temperature regions abo\e latitudes 40° N and 
S there is usually an excess of rainfall over evapora¬ 
tion and consequently positive salinity gradients 
tend to develop beneath the sea’s surface. Along the 
coasts of such regions the outflow from rivers very 
greatly augments this effect and substantial density 
gradients result from the dilution of the iqjper layers 
of water. It follows that the shallow temperature gra¬ 
dients characteristic of temperate regions in summer 
are accompanied by salinity gradients. These gra¬ 
dients are particularly strong in coastal regions. Both 
kinds of gradient cause the water to be more dense as 


ESI 


TEMPERATURE 



SALINITY 

FifiCRi; 3. \’crtical lcni|)eraluie, salinity, and density 

curves in coastal waters. 

depth increases, that is, they supplement one another 
in de^ eloping the stability of the water column. 

It may be observed from Figure 3, which shows the 
gradients of temperature, salinity, and resulting den¬ 
sity in a situation of this sort, that the gradients of 
temperature and salinity very closely coincide iiii 
de|Jth. 4\’here the temperature gradient is strongest 
the salinity gradient is strongest also. I’his arises be¬ 
cause the surface waters are prevented from mixing 
with the deeper waters by the sharp density gradient 
while both abo\e and below, the water mixes more 
freely. Both gradients consequently tend to develop 
in the same relation to the resulting density pattern. 
During the winter, when the disappearance of the 
temperature gradient decreases the stability of the 
water, the mixing which results also destroys the sal¬ 
inity gradient. In spring the melting of snows and the 
rainfall characteristic of the season led to the early 
develojmient of the salinity gradient. This becomes 
relati\ely less important than the temperature gra¬ 
dient in determining the density distribution as the 
summer season advances. 

In the warm oceanic areas salinity gradients are 
less pronounced and are thus of less importance in 
determining the density distribution in the upper 
layers of the sea. In the central basin of the North 
Atlantic, near the borderline between temperate and 
subtropical waters, seasonal changes in evaporation 
and precipitation can produce slight vertical salinity 
gradients at relatively shallow levels. As an example, 

cteQ 














Figure 4. World chart of surface salinities—June, July, August. 
































































































































































































AREAS OF VARIABLE SALINITY GRADIENT NEAR THE SLIRFACE 


53 


DISTANCE IN MILES 

0 100 200 300 400 



the surface salinity in the Bermuda area averages 
close to 36.6 in winter when strong and relatively 
dry w'inds prevail, but in summer the damper, weaker 
winds permit the surface layer to freshen to about 
36.1 '^/oo- The resulting slight increase of salinity 
with depth occurs at about the same level as the 
summer thermocline, so the effect on sound is neg¬ 
ligible. However, for this reason, as well as because 
of the seasonal thermal cycle at the surface, the tem¬ 
perature-salinity correlation is by no means constant 
at depths above the top of the main thermocline. 

In substantial areas of small rainfall in the sub¬ 
tropics evaporation exceeds precipitation, and the 
most saline water is found at the surface. Such sal¬ 
inity gradients are small and are associated with 
similarly small negative temperature gradients, with 
the result that the density of the water column is 
nearly uniform. On the other hand, near the equator 
the high rainfall of the doldrum belt reduces the 
surface salinity and makes it considerably fresher 
than the water near the top of the main thermocline. 
7'he latter, as has been shown above is relatively 
shallow in these latitudes, hence both temperature 
and salinity contribute to the very high stability near 
the surface. 

Thus it is apparent that the distribution of salinity 
in the surface waters shows slight but distinct varia¬ 
tions which are controlled by climatic factors and 
hence have pronounced seasonal and geographical 


characteristics. The latter differ somewhat from those 
of temperature. In general, temperature at the sur¬ 
face decreases with increasing latitude. Surface sal¬ 
inities reach their maximum in a belt centering at 
about latitude 23°, along the outer edges of the trade 
winds where the air is descending and relatively dry 
and where evaporation is greater than either nearer 
the equator or in high latitudes. This is illustrated in 
Figure 4, which is a world chart of midwinter surface 
salinity. 

63 AREAS OF VARIABLE SALINITY 
GRADIENT NEAR THE SURFACE 

\Vhen the surface waters of a warm, saline current 
mix with those of a colder and less saline current, as 
occurs off the Grand Banks of Newfoundland, to cite 
what is probably the extreme example, the tenij^era- 
ture-salinity correlation becomes very variable in¬ 
deed. At times water as fresh as 32 Voo may be just 
above or immediately next to water of salinity greater 
than 36 In the latter case the difference in color 
of the two contrasting water masses may be striking 
and easily seen from a ship’s deck. It is to be noted 
that density may not change much across such a sharp 
line of demarcation because the more saline water is 
also much warmer. 

A similar situation, though less extreme can be 
encountered at any time north of the Gulf Stream 


RESIRICTED 


- 1 

























54 


RELATIONSHIP OF SALINITY AND TEMPERATURE 


between Cape Hatteras and the Grand Banks. Here 
the westerly winds sometimes drive the relati^•ely 
Iresh coastal waters far offshore. I'he situation south¬ 
eastward from Montank Point, Long Island, in July 
1938 (Figure 5) affords a good example of relatively 
sharp salinity gradients near the surface caused by 
coastal water being carried more than 100 miles be¬ 
yond the limits of the continental shelf. Although it 
is just such situations which make it desirable to com¬ 
pensate the submarine-model bathytlier?nograp}i 
[BT] for salinity changes so that it will indicate the 
ballast changes correctly, from the acoustical stand¬ 
point the vertical variations in salinity shown in Fig¬ 
ure 5 are negligible, for in each case the vertical sal¬ 
inity gradients are accompanied by relatively much 
stronger temperature changes. It is necessary to find 
a situation where shallow salinity gradients occur in¬ 
dependently of temperature gradients before sound 
transmission is noticeably influenced. In the open 
ocean this very seldom occurs. However, since one of 
the major assumptions in maximum range predic¬ 
tions based on BT observations is that vertical sal¬ 
inity gradients can be neglected, it is advisable here 
to discuss this matter in more detail. 

The refraction pattern might differ significantly 
from that indicated by a BT trace in the following 
possible circumstances: 

1. A positive temperature gradient might be sig¬ 
nificantly reinforced by an increase of salinity with 
depth. 

2. Within an isothermal surface layer salinity 
might increase with depth, causing increased upward 
refraction. 

3. A slight negative temperature giadient might 
be partially or entirely offset by an increase of salinity 
with depth, 

4. A negative temperature gradient might be re¬ 
inforced by a decrease of salinity with depth. 

6.3.1 Positive Salinity Gradient and 
Positive Temperature Gradient 

During periods of rapid cooling, positive tempera¬ 
ture gradients may be developed near the surface. 
Since this condition is thermally unstable it ordi¬ 
narily occurs only to a slight degree and during calm 
weather. 

"Wherever a marked positive temperature gradient 
exists it is safe to say that there is also a positive sal¬ 
inity giadient which is at least sufficient to counter¬ 


balance the thermal instability of the water column. 
The combination of positi\e temperature and salin¬ 
ity gradients ma) occur when there is a heavy rain 
that freshens the surface layer, and when the rain is 
colder than the sea water or is followed by conditions 
that favor surface cooling by evaporation. 

Still more marked examples are found near the 
edge of a current or along the continental slope where 
there is a flow of colder o\er warmer and more saline 
water. In such cases the density gradients are often 
so highly stable as to permit a submarine to balance 
on what would appear from the temperature trace 
alone to be an unstable la) er. 

6.3.2 Positive Salinity Gradient 

in Isothermal Layer 

1 his situation can and docs occur occasionally in 
the open ocean. It is relatively common close to the 
land in early spring. However, offshore the resulting 
increased upward refraction is seldom critical to 
sound conditions, nor is the resulting stability of the 
water column often sufficient to influence trim sig¬ 
nificantly in diving. 

In the open ocean salinity can increase with depth 
within a virtually isothermal surface layer during 
and after a heavy rain, pro\ ided the winds are light 
and provided the temperature of the rain water is 
close to that of the surface water. The circumstances 
are probably most frequently favorable in the dol- 
drum belt. 

6.3.3 Positive Salinity Gradient and 
Negative Temperature Gradient 

This situation could occur when slight surface 
warming is accompanied by precipitation. 

In the current range-prediction method the mini¬ 
mum negative temperature gradient that is signifi¬ 
cant is a decrease of more than 0.3 degree between the 
surface and a depth of 30 feet. For the downward re¬ 
fraction thus produced to be counteracted, the posi¬ 
tive salinity gradient would have to average more 
than 0.4 o/oo to Voo over the same depth range 
(see Figure 3, Chapter 3), depending on the surface 
temperature. It is unlikely that this ever occurs in the 
open ocean, except possibly in nearly calm weather 
when the negative temperature gradient persists for 
several days. Sufficient precipitation is not apt to take 
place during the few hours Avhen slight negative tem- 


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AREAS OF VARIABLE SALINITY GRADIENT NEAR THE SURFACE 


55 


perature gradients become established by diurnal 
^vanning. 

6.3.4 Negative Salinity Gradient and 
Negative Temperature Gradient 

A more likely case is that surface warming is ac¬ 
companied by evaporation, the negative salinity gra¬ 
dient being insufficient to counteract the effect of the 


negati\e temperature in maintaining stability. In 
this case the downward refraction due to tempera¬ 
ture would be reinforced slightly by the salinity gia- 
dient. It is believed that this effect is common in such 
specialized situations of dry, warm air as occur in 
parts of the Mediterranean, the Red Sea and the 
Persian Gulf. However, the further reduction in 
range which can occur in this way is of minor prac¬ 
tical importance. 


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GEOGRAPHICAL AND LOCAL VARIABILITY 


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1 ‘ 


it 


•■V 





•m 



Chapter 7 

OCEAN CURRENTS 


T he factors considered up to this point are those 
for the most part wdiich ha\ e an orderly and gen¬ 
erally gradual effect on temperature and salinity 
structure. A picture has been presented of a more or 
less idealized ocean with a temperature structure 
which is permanent and relatively constant except 
near the surface where small-scale heating, cooling, 
and mixing processes result in diurnal and seasonal 
temperature changes. 

However, it has gradually become apparent that 
there are important geographical variations in tem¬ 
perature structure which are caused partly by cli¬ 
matic variations and partly by the great ocean cur¬ 
rents. Some knowledge of currents, both as to their 
general characteristics and the location of the major 
systems, is essential for mastery of the problems of 
subsurface warfare. 

Variability of a more local nature must also be 
considered. Under this heading are included a wide 
variety of phenomena. The surface processes of heat¬ 
ing and cooling are subject to a great deal of local 
variation, particularly in temperate and polar re¬ 
gions, where relatively sudden and violent weather 
changes are common. Various kinds of small-scale 
thermal structure are produced as a result of water 
transport or meteorological influences. Finally there 
are the special phenomena of coastal waters. All such 
variability not only is an essential part of ocean¬ 
ography but also plays an important role in sound 
transmission. 

7.1 PRINCIPLES OF 

OCEAN CIRCULATION 

7.1.1 Water Movements in Relation to 
Density Distribution 

Previous sections have mentioned the movements 
of water that result from changes in density. In the 
tropics where surface heating is strong, the light 
water thus formed tends to spread northward and 
southward over the surface and to be replaced by 
colder water from below. As it moves into high lati¬ 
tudes, it overlies water that is colder but less saline, 
liaving been subjected to less heating and less evap¬ 


oration. Thus when the tropical water cools, its salin¬ 
ity makes it denser than the water underneath, and it 
sinks. This surface transfer of water toward the poles 
is compensated l)y a return flow which largely takes 
place as a slow drift in the depths of the ocean. Simi¬ 
lar movements also take place on a much smaller 
scale as a result of local heating and cooling. 

In discussing the forces involved in such move¬ 
ments, it is convenient to compare them with meteor¬ 
ological phenomena, since the problems are essen¬ 
tially the same. A weather map shows isobars, lines of 
ecpial barometric pressure separating low- from high- 
pressure areas or topographic contours of an isobaric 
surface. In the sea as in the atmosphere, pressure in¬ 
creases downward because of the weight of the over- 
lying layers, and the pressure at a particular depth 
will vary according to the density of the overlying 
medium. It is therefore possible to chart the pressure 
distribution in the sea (usually by plotting the topog¬ 
raphy of a selected isobaric surface) which will show 
the direction of water movement in the same way 
that the isobars on a weather map indicate wind di¬ 
rection. The usefulness of these topographic charts 
depends on the fact that the pressure gradient is zero 
in the direction parallel to the contours. Therefore, 
the initial tendency is for the water to flow at right 
angles to the contours, that is, downhill. As soon as 
the water is set in motion, however, it is deflected 
from its downhill course by the effect of the earth’s 
rotation, as will be explained below'. As long as the 
isobaric slope is maintained on a rotating earth, the 
current w ill run more or less jjarallel to the contours 
of the isobaric surface in the same way that the wand 
follow's the isobars on a w'eather map. In order to 
understand this situation fully, it is necessary to ex¬ 
amine in detail the effect of the earth’s rotation, 
which will be taken up in the next section. It will 
also be necessary in dealing with water movements in 
general to discuss the important role played by wind¬ 
generated currents. 

Coriolis Force 

The effect of the earth’s rotation on moving bodies 
is known as Coriolis force, after the French physicist 


RESTRIC FED 




.'iO 




()() 


OCEAN CURRENTS 



FiciiiRK 1. Dia<^iani illustrating Coriolis force—rotation of 
points near the North Pole. 


Fioure 2. Diagram illustrating Coriolis force—a|jparent 
motion of a swinging pendulum on a rotating earth. 



who evolved the mathematical theory. It is a ficti¬ 
tious force, as will be explained, and acts always at 
right angles to the direction of motion. Its effect is to 
make a current of Iluid particles seem to be deflected 
toward the right in the Northern Hemisphere and to 
the left in the Southern Hemisphere. Only at the 
ecjuator is Coriolis force nonexistent, and it increa.ses 
to a maximum at the poles, varying as the sine of the 
latitude. 

I’o explain how this force acts, suppose an ob- 
.server is standing at the North Pole in the center of a 
horizontal circidar platform and is looking at a point 
on the edge of the platform. In Figure 1 the position 
of the observer is at A, and he is looking at point B. 
d'he earth rotates in the direction indicated, and 
jK)int B rotates with it, taking the successive posi¬ 
tions Bi, B 2 , B 3 , and returning to the original posi¬ 
tion of B in 24 hours. The observer, of course, turns 
as the earth turns, continuing to face B, and is not 
aware that the position of the latter has changed. It 
would be apparent to him, however, that some such 
motion was taking place if he built an instrument 
that was capable of moving in straight lines across 
the platform without being afiected by the earth’s 
rotation. Such an instrument would be a pendulum 
suspended from a tower at A and swinging toward 
and away from B, as indicated by the dotted line in 
the figure. It woidd continue to swing in this plane, 
but B would rotate away from it until at position Bj 
the pendulum would swing in a line at right angles 
to the direction of Bi from A. To an outside obser\er 
it is obvious that the pendulum is swinging in a 


straight line, but to the observer at A it woidd look 
as if B stands still and the pendulum swings more 
and more to the right. If the pendidum moved slowly 
enough so that its course on each swing could be 
[dotted, it woidd appear to be mo\ ing in a series of 
curves, each one bent slightly to the right of the di¬ 
rection of motion as in Figure 2. After several swings 
its direction as shown in the figure would be about 
at right angles to a line from A to B, and it would 


N 



S 


FifiiiRE 3. Diagram illustrating Coriolis force—rotation of 
points on the ecjuator. 


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PRINCIPLES OF OCEAN CIRCULATION 


61 



continue to change, making one complete apparent 
rotation in 24 hours. 

A water mass at A or B will rotate with the earth 
and will therefore appear stationary to an observer at 
either spot. But if some force puts the water in mo¬ 
tion in any horizontal direction it will behave simi¬ 
larly to the pendulum, that is, it will tend because of 
its momentum to move in a straight line so that the 
earth rotates under it and its path appears curved to 
an observer who is not aware of the earth’s rotation. 
Thus a current with an initial impulse in the direc¬ 
tion from A toward B will curve to the right and will 
pass to the westward of B. 

In the same way a perfectly aimed bullet will strike 
to the right of the bull’s-eye (in the Northern Hemi¬ 
sphere), whatever the direction between the gun and 
the target may be. While the bidlet is in the air the 
earth has twisted under it slightly so that the initial 


bearing of the target in relation to space has changed, 
riius any body which has been set in motion and is 
not acted upon by any lateral force appears, because 
of the earth’s rotation, to be moving in a circular 
path, which is called its inertia circle. The radius of 
curvature depends on the velocity. The bullet ap¬ 
pears to curve only slightly, but an ocean current 
with a much lower velocity has a smaller radius of 
curvature. 

If point A were located at the South Pole, the 
earth’s rotation would carry point B around it in a 
clockwise instead of counterclockwise rotation, hence 
the apparent deflection of a pendulum or of a cur¬ 
rent of water is to the left. This general rule for the 
direction of deflection holds for the lower latitudes 
as well as the higher; the deflection is to the right in 
the Northern Hemisphere and to the left in the 
Southern. The amount of deflection becomes less. 


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62 


OCEAN CURRENTS 






Figure 5. Diagram illustrating Coriolis force—rotation of points at 45° N. 


however, at lower latitudes and ceases at the ec[uator. 
Suppose points A and B are on the ecpiator as in 
Figure 3. As the earth rotates, B lollows A with no 
change in horizontal position. There is still a circling 
motion of a sort, which can be visualized by holding 
A in a fixed position and letting the whole earth ro¬ 
tate around it for a day’s time as we look down on it 
from the North Pole (Figure 4). But to the observer at 
point A the circling motion is in a vertical plane, and 
it does not result in any horizontal deflections in the 
water around him. 

If the points are located at an intermediate lati¬ 
tude, say 45° N, then the circling motion has some 
of the characteristics of both the polar and ecpiatorial 
movements, in that it has both horizontal and verti¬ 


cal comjjonents. The resultant movement in a hori¬ 
zontal ])lane around the ol)scr\cr at 45° N is impor¬ 
tant. It is not quite so easy to visualize B circling 
around A at this latitude as it was at the Pole. In 
order to simplify the illustration. Figure 5 is drawn 
as if A is stationary and B is circling around it. Actu¬ 
ally, of course, both points are in motion, circling 
around each other. 

One of the most difficult things to understand 
about Coriolis force is the fact that although the 
earth rotates once every 24 hours, except at the North 
and South Poles it takes more than 24 hours for the 
jioints A and B to circle each other, and for this rea¬ 
son also. Figure 5 is an o\ ersimplification of the facts. 
This was proven by Foucaidt, the physicist who first 


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PRINCIPLES OF OCEAN CIRCULATION 


63 



Figure 6. Diagram illustrating Coriolis force—rotation of 
points at 45° X. 

demonstrated the pendulum experiment described 
above. Going back to that discussion, there was a 
circular platform at the Pole, rotating with the earth 
so that the pendulum was swinging above it in a 
straight line but apparently making a complete rota¬ 
tion in 24 hours. Suppose now the platform is enor¬ 
mously enlarged and tilted so that its edge reaches 
points A and B on latitude 45° N. It still must be 
horizontal at points A and B because we are inter¬ 
ested only in horizontal deflections. It must rotate 



Figure 7. Diagram of water movements resulting from 
localized heating of the sea surface. 


with the earth, so its center is the prolonged axis of 
the earth. Thus it is as shown in Figure 6. This disk 
then is the plane across which the pendulum swings. 
It rotates as the earth does with its edge touching at 
points A and B. But its circumference is greater than 
the distance around the earth at latitude 45°; there¬ 
fore, it is similar to a reduction gear, with one revo¬ 
lution of the disk recpiiring more than a complete 
revolution of the earth. But points A and B circle 
each other as the disk circles and the plane of the 
pendulum shifts in the same way, both requiring 
longer than 24 hours to complete their rotation.® As 
the disk is further enlarged in order to extend to 
lower latitudes, the circling motion becomes slower. 
Finally at the ecpiator it is impossible for a disk hori¬ 
zontal with the surface of the earth to have the axis 
pass through it. In other words, the length of the 
circumference of the disk is infinity and the circling 
motion is zero. The apparent deflection of the pen¬ 
dulum or of a current of water varies with the speed 
of rotation of the disk and its points A and B. There¬ 
fore Coriolis force is zero at the ecpiator and maxi¬ 
mum at the poles, and it can be proven mathemat¬ 
ically that the variation is proportional to the sine of 
the latitude. 

7.1.3 The Circulation Theory 

It is apparent now that since any movement of 
water is affected by the earth’s rotation, the simple 
explanation of circulation presented in the previous 

a The length of time required for a com25lete rotation of the 
plane of the pendulum at any given latitude is called a pendu¬ 
lum day. The time recjuired for a moving body at the same lati¬ 
tude to complete its inertia circle (see p. 61) is one-half of this 
time or one-half pendulum day. The reason for this may not be 
immediately apparent from the foregoing discussion. some¬ 
what more advanced treatment of the stibject will be found at 
tbe beginning of Chapter XIII of reference 1, Chapter 1. 



Figure 8. Equilibrium that results when movements 
shown in Figure 7 are carried to completion. 


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64 


OCEAN CURRENTS 



Figure 9. Effect of Coriolis force on water movements 
resulting from localized heating of the sea surface. 


section requires some modification. In building a 
theory that will explain the principles of circulation, 
it is desirable for the sake of simplicity to progress 
one step at a time, showing how each of these forces 
plays its part in setting up ocean currents. But in so 
doing it must be understood that each step is an ideal 
case that has, with minor exceptions, no counterpart 
in actual water movements which are affected b)' all 
the forces at once. At the end it will be seen that they 
fit together into a usable picture of the major current 
systems of the world. 

First of all. Figures 7 and 8 show the effect of den¬ 
sity changes on a nonrotating earth. I'he surface 
water in a particular area is heated until it is warmer 
than the water around and underneath it. Heating 
expands the water so that the surface of the sea is 
slightly elevated in the center of the warm spot, and 
immediately the water begins to flow downhill in all 
directions away from the center. Colder and denser 
water then presses up against the warmer and lighter 
water, buoying it up. Circulation therefore occurs as 
indicated by the arrows in the diagram, and it con¬ 
tinues until there is an even distribution of density 
and pressure (Figure 8). 

The effect of the earth’s rotation is to cause all 
these movements to be deflected to the right (in the 
Northern Hemisphere). The result is a whorl of 
clockwise water movements spreading out from the 
warm center, as shown in Figure 9. Under ordinary 
circumstances this eddy of warm water can be ex¬ 
pected to spread until the density differences are 
equalized and the condition of Figure 8 is estab¬ 
lished. The process is retarded, howe\er, by the ten¬ 
dency of water particles to be deflected more and 
more to the right, so that at the edge of the eddy the 
movement follows very nearly a circular path. 

I'he sinking of cold or saline water produces a 
somewhat similar current pattern but in the opposite 



Figure 10. Diagram of water movements resulting from 
localized cooling of the sea surface. 


direction. If the surface water in a particular area is 
cooled stifficiently to make it denser than the water 
underneath, it will sink until it reaches a level of its 
own density. As it does so the sea surface is depressed, 
and warm surface water flows in from all sides to take 
its place. Coriolis force transforms these movements 
into a counterclockwise whorl (in the Northern 
Hemisphere) as shown in Figure 10. At first glance 
these water mo\ements appear to be deflected to the 
left rather than the right. That this is not so is demon¬ 
strated by Figure 11, which shows the movement of a 
particle of water from the edge of the warm water (A) 
to the cold center. At each instant of its progress. A, 
Ai, Ao, etc., the direction of force is toward the center 
as indicated by the dotted lines, while the direction 
of movement as modified by Coriolis force is to the 
right of the direction of pressure. 

Suppose now the conditions of Figures 9 and 10 are 
combined in a continuous process. For example, the 
eddy of warm water is continually heated too rapidly 
for water movements away from the center to stabil¬ 
ize the density distribution completely. Then at a 
point on the periphery of the eddy the water is cooled 
just as rapidly as it was heated. ^Vater will flow from 
the first point to the second, and it is conceivable that 



Figure 11 . Movement of water into a cold center, show¬ 
ing direction of jrressure and the resulting motion as 
modified hv Coriolis force. 


(RTSTRJCrro 
___ 























PRINCIPLES OF OCEAN CIRCULATION 


65 



Fif.iiRE 12. Movement of water between two localized 
areas, one of which is l)eing warmed, the other cooled. 



a permanent eddy of constant size and shape will be 
established; this is the result of a balance between the 
force produced by the density distribution and the 
frictional resistance of the water to this force. This 
situation can be visualized diagrainmatically in Fig- 
nre 12, in which point A is the center of heating, and 
the warm water is cooled at point B. The water 
spreads out from A in all directions in the form of a 
clockwise whorl as previously described and then 
converges at B. As long as the rates of heating and 
cooling remain constant the pattern of currents and 
the size of the eddy will also be constant. This condi¬ 
tion of equilibrium is commonly known as a steady 
state. In parts of the eddy the density distribution is 
in a permanently unstable condition, particularly 
near the edge where there is a sharp downward slope 
in the thermocline. In other words the inecpiality in 
density structure still generates pressure which is di¬ 
rected outward from the center of the eddy in all di¬ 
rections. However, the water movements that must 
be produced as a residt of the pressure gradient do 
not increase the size of the eddy; instead they take 
a ciretdar clockwise course at its periphery. This is 
the extreme example of the application of Coriolis 
force. Just as Coriolis force is proportional to the 
mass and velocity of the movement of water and at 
right angles to it, so it exactly balances the pressure 
that jii'oduces the water movements and is at right 
angles to the direction of pre.ssure. Thus, when the 
presence of a permanent slope in the density surfaces 
indicates that a steady state has been attained, the 
current that is generated is always at right angles to 
the direction of pressure, so that if an observer in the 
Northern Hemisphere faces in the direction in which 
a current is flowing, the density surfaces slope down¬ 
ward toward the right. 

I’his balance between the j)ressure gradient and 
Coriolis force can perhaps be more easily understood 


by recalling the inertia circle described in the preced¬ 
ing .section. It was stated that a particle in motion and 
not under the influence of any lateral force appears 
to take a curved path because the earth is turning 
beneath it. The radius of the circle varies with the 
velocity and the sine of the latitude. For a current of 
average velocity located in mid-latitudes this radius 
is about 15 miles. But the major ocean currents ob¬ 
viously have a very much larger radius of curvature, 
which means that they must be under the influence 
of a lateral force which deflects them from the path 
of the inertia circle. In other words, for water to flow 
in a reasonably straight path in the geographical 
sense, it must be acted upon by a component of gravi¬ 
tational force (the crosscurrent pressure gradient) 
which deflects it to the left in the Northern Hemi¬ 
sphere and to the right in the Southern just enough 
to balance Coriolis force. 

In a very general way the description thus far de¬ 
veloped can be applied to ocean circulation. AVeak 
density gradients extending over great horizontal 
distances are important in transporting warm trop¬ 
ical and subtropical water to polar regions. AVithin 
the major ocean currents where steady-state condi¬ 
tions are approximated at least through parts of their 
courses, the flow is very nearly at right angles to the 
pressure gradient. However, the uneven nature of 
the processes that originate the currents leads to con¬ 
tinual variations in velocity and varying amounts 
of crosscurrent transfer which shifts their position. 
Near the surface the effect of pressure gradients is 
also greatly modified by the wind systems of the earth. 
Around each of the central basins of the oceans the 
winds form a gi'eat anticyclonic eddy, consisting pri¬ 
marily of the trades and prevailing westerlies. The 
force of these winds produces similar eddies in the 
ocean, the general features of which are illustrated 
in Figure 13. 


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66 


OCEAN CURRENTS 


Movement of water in these eddies is deflected by 
the earth’s rotation to the right of the wind direction, 
hence there is a drift of water toward the center of the 
eddy accompanied by a slight elevation of the sea 
surface where the water is converging. This creates 
an unstable pressure distribution that is compensated 
in two ways: (1) by a return drift toward the peri- 
phei')' of the eddy in the water underneath the wind- 
driven surface layer, and (2) by a general redistribu¬ 
tion of density structure to equalize the vertical pres¬ 
sure. In other words there is a tendency for readjust¬ 
ment of density surfaces so that the pressure on the 
underlying deep water caused by the weight of Avater 
above it is uniform, and any tendency to increased 
weight due to elevation of the sea surface is offset by 
an increase in depth of the relatively light mixed 
layer. Thus the light water accumulates in the center 
of such a convergence, and the density surfaces slope 
doAvn toward the center from all sides, giving rise to 
a permanent lens-shaped body of warm water. 

This situation is stable only insofar as it is main¬ 
tained by the force of the wind. If the latter ceased, 
the warm water would sjjread out in an even layer 
over the surface in the same manner as described in 
Figures 7 and 8. In this respect it is similar to an eddy 
of warm water maintained by surface heating, which, 
as has been seen, is permanent only by the action of 
an external agency. 

The similarity does not end there, however. In 
each case there is an elevation of the sea surface and 
deformation of density surfaces, resulting in the de¬ 
velopment of a pressure gradient which in turn pro¬ 
duces a current flowing at right angles to it. All these 
factors are interrelated—the difference in density be¬ 
tween the warm water above and the ct)lder water 
below, the slope of the thermocline (as influenced by 
latitude), and the velocity of the current produced. 
The relationship can be expressed by a mathematical 
formula, so that by measuring the vertical density 
structure of ocean Avaters, it is possible to determine 
the direction and Aelocity of currents. Current sys¬ 
tems are commonly studied in this Avay, for actual 
measurements of the currents are Aery inaccurate un¬ 
less the vessel is equipped for anchoring in deep 
Avater, and even so they are difficult and laborious. 
The principle of determining ocean currents from 
the density structure of the sea, Avhich is generally 
knoAvn as the Bjerknes Circulation Theory, is one 
of the most important contributions to modern 
oceanography. 


But as indicated above, Avhile the mathematical 
relations of currents Avith the density structure of the 
Avater are ahvays about the same, the causal relation¬ 
ships are often inextricably mixed. The Avind may 
set iqj a current, and the transport that results Avill 
cause the density surfaces to slope. ContrariAvise, den¬ 
sity slopes that result from heating or cooling proc¬ 
esses Avill cause a current to be formed. Often both 
processes are iiiAolved, and some of the most poAver- 
ful ocean currents are tho.se in Avhich the prevailing 
Avinds and the density distribution Avork together. 

Wind Transport 

In the open ocean many of the permanent currents 
move in roughly the same direction as the prevailing 
Avinds and are primarily Avind-driven. It is obvious 
that these currents are deep and poAverful and that 
huge amounts of momentum are iiiAohed. But such 
basic (juestions as hoAV long a Avind must bloAV before 
a current becomes established and in Avhat direction 
this current Avill flow still must be dealt Avith largely 
on the basis of theory, for feAV satisfactory field studies 
have been made. 

The classical theory of Avind currents Avas adA anced 
largely on the basis of mathematical calculations, and 
although it has since been refined and modified, 
it is best to begin by discussing it in its simple 
form. The theory assumes a limitless, deep ocean in 
Avhich there is no change in density Avith depth. The 
force of the Avind sets up a drift current in the surface 
Avater. As soon as the Avater is set in motion it is acted 
upon by Coriolis force, and Avhen steady-state condi¬ 
tions are reached the resultant current at the surface 
is 45 degrees to the right of the doAviiAvind direction 
in the Northern Hemisphere and to the left in the 
Southern. I'he Avater beloAv the surface is dragged 
along by friction, the Aelocity decreasing Avith in¬ 
creasing depth, and the direction of motion swinging 
more and more to the right. This is the so-called 
Ekman spiral (Figure 14), named after the author of 
the theory. If the degree of frictional drag, or so-called 
eddy A'iscosity, is constant Avith depth as assumed in 
the calculations, then it can be shoAvn mathemat¬ 
ically that the net transport of the Avind-driven sur¬ 
face layer Avill be 90 degrees to the right of the Avind 
(in the Northern Hemisphere). It is believed that this 
sort of motion Avill become established Avithin about 
12 hours of the onset of a steady Avind. Earlier the 
floAV Avill be more nearly in a doAviiAvind direction. 


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67 


PRINCIPLES OF OCEAN CIRCULATION 



Figure 14. Diagram illustrating the Ekman spiral. 


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68 


OCEAN CURRENTS 


I'his situation is complicated by the fact that wind 
transport is always accompanied by wave transport. 
Furthermore it is associated with a considerable 
amount of vertical turbulence, frecpiently with the 
formation of convection cells of the kind described in 
Section 5.2.1, which produce some mixing to the 
bottom of the isothermal layer. Moreover, it has been 
observed that in the convergences of convection cells 
the surface water moves downwind at a considerably 
more rapid rate than in the divergences. This can 
sometimes be seen on a cold day by observing the 
bubbles and foam on the sea surface. Since the flow 
of water in response to the wind is uneven and since 
mass vertical motions occur in the convective layer, it 
is clear that the mathematical assumptions oversim¬ 
plified the actual movements of water in the wind- 
driven surface layer. 

Furthermore, when water is transported by the 
wind, other water tends to take its place (compensat¬ 
ing currents), while ahead of the current, water tends 
to be pushed forward and piled up, leading to a slight 
elevation of the sea surface and a depression of the 
thermocline to the right of the wind. This causes the 
development of a deeper current, commonly known 
as a gradient current, according to the principles of 
the circulation theory, which flows at mid-depths 
parallel to the wind direction and below the Ekman 
spiral. When compensating and gradient currents 
have been set up, they .seriously affect the direction of 
the original wind-driven current, and the wind-drift 
theory in its simplest form is no longer completely 
correct. Thus it is argued that when a steady state has 
been reached, the deflection of the surface water from 
the downwind direction will be reduced by perhaps 
half the 45 degrees called for by the simple theory, 
and indeed in the trade wind belts, at least, it is pos¬ 
sible to demonstrate that the current at the surface is 
at an angle of roughly 15 to 20 degrees to the right of 
the wind. 

For the present purposes, however, what is particu¬ 
larly important to subsurface warfare is not so much 
the cpiestion of the exact direction of flow or velocity, 
but rather a general understanding of the way in 
which local variability of the winds can transport the 
surface water and sometimes carry warmer water over 
colder and vice versa, thus radically affecting sound 
conditions. Moreover, wind transport and the asso¬ 
ciated gradient currents may in some cases move con¬ 
siderable masses of the warm surface layer, piling it 
up and thickening the isothermal layer (convergence) 


or spreading it out and reducing the amount of 
mixed water (divergence). 

The effects in the open ocean of convergence and 
divergence on a large scale are evident on the sum¬ 
mer-season sonar charts (see Figures 9 and 10 in 
Chapter 5). In the latitude of westerly winds, for ex¬ 
ample, the sound conditions are generally better on 
the lee side of the ocean than on the windward. Like¬ 
wise, in the trade wind belt sound conditions in the 
west are relatively good, for example in the Carib¬ 
bean. 

72 MAJOR CURRENT SYSTEMS 

OF THE EARTH 

Figure 15 is a chart of the major surface currents of 
the oceans. For the most part their position shifts 
only slightly with the seasons, although in the north¬ 
ern part of the Indian Ocean and along the China 
coast the direction of their flow is actually reversed. 
Here they are shown in their midwinter position. 

If Figure 15 is compared with previously published 
current charts a marked difference will be noted. 
Most current charts show the average direction and 
strength of all available drift measurements for each 
small area of the ocean, usually supplemented by in¬ 
ferences from the more numerous wand observations. 
As a resvdt they make the currents appear broad and 
diffuse and closely dependent on prevailing local 
winds. It is not always easy from such a chart to dis¬ 
tinguish clearly between the central portions of the 
major eddies, where the direction of drift is relatively 
haphazard, and the edges of those eddies where one 
finds true currents involving significant transport in 
a nearly constant direction and enough momentum 
to leave them unchanged by occasional periods of 
opposing winds. 

An attempt has been made in Figure 15 to show by 
the continuous lines where the direction of flow is 
most persistent, and to group these lines into bands 
outlining the edges of eddy movement in order to 
indicate where the currents are most clearly defined. 
The spacing of the lines cannot, however, be taken 
as an exact measure either of constancy or of speed, 
and it shonld be noted that the Mercator projection 
makes them appear widely separated in the AVTst 
Wind Drift, which is actually a fairly strong current. 

The courses and speeds of the currents at the sur¬ 
face are well known through ships’ observations, and 
oceanographers have studied the flow at greater 


RESTRICTED 





































































































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I’i(;i)UK 17. \V'oi l(l chai l sliowiiig the prevailing winds in Angnst. 
































































































































































































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I 




MAJOR CURRENT SYSTEMS OF THE EARTH 


69 



Fk;uri; 18. Diagram of topical ocean circulation, wind- 

driven eddv and sidipolar convergence. 

depths, partly by direct observation and to a larger 
extent by determinations of the density structure. 
The velocity of the currents is greatest near the sur¬ 
face where in a few places they reach a speed of 3 or 
4 knots. The greatest depth at which a significant 
amount of flow can be detected varies with each cur¬ 
rent and in diflerent parts of the same current, but in 
some cases this depth is greater than a mile. It is ap¬ 
parent therefore that the larger currents carry huge 
volumes of water. For example, the total transport of 
the Gnlf Stream in the latitude of Chesapeake Bay is 
approximately 100 million tons a second. 

In examining the currents it is worth while to com¬ 
pare them with the wind systems of the world (Fig¬ 
ures 16 and 17), which are similar in many respects 
and are to a large degree responsible for the general 
pattern of the currents. This is particnlarly true of 
the west wind drift in the Antarctic and of the North 
and South Ecjnatorial Currents vvOiich lie in the trade 
wind belts. The latter currents form part of the great 
eddies centering in mid-latitndes, clockwise in the 
Northern Hemisphere, and counterclockwise in the 
Southern, corresponding to the prevailing wind di¬ 
rection. Since they are primarily wind-driven, they 
are swiftest at the surface. In the main thermocline 
and the upper part of the deep water layer, horizontal 
eddy-type of motion gradually decreases with depth, 
giving way to slower deep water transport. 

These eddies take the general form described in 
the preceding section, and together with sinking 
centers in higher latitudes they set the dominant pat¬ 
tern of the current systems of each ocean basin. Fig- 
nre 18 is a diagrammatic representation of the scheme 


of circnlation, showing the density structure and the 
direction of the currents in a combined wind-driven 
eddy and sinking center. 

It is important to consider the density structure of 
the oceans in relation to the currents, since it is not 
only essential for proper understanding of the sub¬ 
ject but also has practical implications from the 
standpoint of echo ranging. For this purpose refer¬ 
ence is made to previous figures (9 and 10, Chapter 5) 
which showed layer depth contours taken from the 
sonar charts for December to February and June to 
August. These are in a sense a superficial picture of 
the density distribution of the sea, but they are ade- 
([uate for the present purposes. Winter conditions 
(Figure 9, Northern Hemisphere; Figure 10, South¬ 
ern) are particnlarly useful for examining the density 
structure in relation to currents. 

In the eddy currents centering in mid-latitudes, the 
density surfaces slope downward toward the right in 
the Northern Hemisphere and the left in the South¬ 
ern as is expected according to theory. Thus in the 
center of the eddies, layer depths are greater than 
around the outer edges of the currents, and the win¬ 
ter sound ranges are correspondingly better. On the 
edges of the currents the warm water is often found 
as a shallow wedge over colder and less saline water. 
This condition is further shown by bathythermo¬ 
grams of the Florida Current and of the Culf Stream 
further north (Figures 19 and 20). The negative tem¬ 
perature gradients at the edge of the current make 
echo ranging conditions poorer than they are either 
in the main body of the current to the right or in the 
colder slope water, as it is called, to the left. With 
northerly or westerly winds the slope water is forced 
to overlie the warmer and more saline Cnlf Stream 
water disrupting the normal temperature structure, 
producing temporary positive gradients, and setting 
up eddies that lead to complex and variable tempera¬ 
ture conditions. For all these reasons the edges of 
such a current are poor and variable from the anti- 
sidjinarine standpoint. 



Figcrk 19. Series of bathythermograms across the Florida 
Current. 


restricted] 


























OCEAN CURRENTS 


70 



COASTAL SLOPE WATER GULF STREAM 

WATER 


Fkjiirk 20. Series of bathytlieiniograins across the Ciulf Stream. 


Ill Slimmer (Southern Hemisphere, Figure 9; 
Norihern, Figure 10 of Chapter 5) the development 
of the seasonal thermoclines obscures this general 
jiicture of density structure in the central basins, as 
shown in the sonar charts. 7’hc current systems and 
their rclationshiji to the main thermocline are ap¬ 
proximate!} the same, Imt they do not benefit sound 
ranging conditions in the center of the great eddies, 
dlierc, as well as in the waters beyond the edge of the 
currents, seasonal warming shortens sound ranges. 
AV'here the current is strongest, the turbulence de- 
\eloped aids the vertical distribution of heat, hence 
the best sound ranging conditions in the summer are 
found in areas where there arc currents. This is par¬ 
ticularly true of currents that are partly or largely 
wind-drii en so that the effects of wind-generated tur¬ 
bulence and turbulent flow are comliined. However, 
sonar conditions are also good in such places as the 
Florida Straits where the wind is light and variable in 
summer and does not affect the current ajipreciably. 

Whthin the North and South Etpiatorial Currents 
the density surfaces slope down toward the north in 
the Northern Hemisphere and to the south in the 
Southern Hemisphere (Figure 21), and water is trans¬ 
ported not only westward but also away from the 
eejuator on both sides. Therefore where these cur¬ 
rents border on the equatorial doldrtim belt the 
thermocline is shallow, and hence echo ranging con¬ 
ditions are relati\ely poor in the doldrums. At the 
same time the steady blowing of the trade winds 
exerts enough stress on the sea surface to raise the 
sea lc\ el slightly (in the Atlantic ocean it is about 2f4 
inches per thousand miles) in a westward direction 
along the etpiator. 1 he etpiatorial countercurrents 
lound in the doldrums result as a downslope How iii 
the /one tvherc the 'winds maintaining the slope arc 
absent. 


In the subarctic regions of both the Atlantic and 
the Pacific oceans, the dominant feature is a group 
of counterclockwise currents set up around the so- 
called sinking centers. The depression of the density 
surfaces and the thickening of the mixed layer that 
accompaii} these convergences make .sound ranging 
conditions good except when strong winds interfere, 
or when lateral eddies between water masses of dif¬ 
ferent temperature and salinity produce confused 
negative and positive gradients. 

In the Southern Hemisphere, sinking of surface 
water cleri\ed from the tropics also occurs, but be¬ 
cause of the strong circumpolar currents which arc 
caused by the pre\ailing westerh winds and are un¬ 
impeded by the presence of land masses as in the 
Arctic, sinking occurs in a more or less continuous 
band at a latitude of about 50 to 60° S, rather than in 
localized centers. From this convergence southward, 
layer depths arc almost invariably great enough for 
good echo ranging, but the effecti\’c sound range is 
frct[uently redticetl by strong winds. 

The acTuaf dilfercnces in density in the converg¬ 
ences are generally slight, and while presumably a 
certain amount of sinking of hea^ ier water occurs all 
the time in one part of the area or another, it seems 
jirobable that it is a highly variable phenomenon, 
much inllucnced liy local water movements and 
weather. Ordinarily sinking occurs at the boundary 
between two water masses of slightly different tem¬ 
perature and salinity. Pronounced downward move¬ 
ment of water is favored by a wind that transports 
heavy water oxer a mass of lighter water, or by cold, 
calm xveather that permits considerable masses of nn- 
stable xvatcr to dexelop at the surface before they 
liegin to sink. 

No measurements have been made of the doxvn- 
xvard currents that result, and there is onlv the 


RESTRICIEI^ 






















MAJOR CURRENT SYSTEMS OF THE EARTH 


71 


LAT 2° S. 0° N. 2° 4 6'’ 8" 12° 14° 16° 



^ SOUTH EQUATORIAL CURRENT EQUATORIAL COUNTER CURRENT NORTH EQUATORIAL CURRENT 

^- (FLOWS WEST) --- (FLOWS EAST) -- (FLOWS WEST) - 

I 



Figi'rk 21. Profiles showing isolhcnns across tlic l-'(|uatorial Currents in the Pacific. 


xaguc-sl inlonnation about their size and form, l)ut 
e\ idence of their existence has been observed from 
surface \essels. Sinking ordinarily occurs in a hand 
a few yards wide and perhaps as much as several 
miles long, ft has the choppy appearance of a tide 
rip and may he full of seaweed and other debris 
brought into it by the con\erging surface currents, 
rhe downdraft is strong enough to carry down debris 
of slight buoyancy, and the surface currents may he 
strong enough to hold a drifting vessel in the converg¬ 
ence in spite of a cro,sswind. 

Such con\crgcnccs are probably important from 
the standpoint of di\ ing o|3erations. Submariners use 
the term fresh pocket for a place where the vessel 
suddenly and for no oln ious reason begins to sink 
and requires reballasting. This can happen any time 


when a sidnnarine goes too near the lower limit of a 
supporting density gradient. Varying thickness and 
amount of temperature change in the thermocline is 
no doubt responsible for some of the instances re¬ 
ported. On the other hand, sometimes the submarine 
sinks very rapidly and checks its descent with great 
difficulty and at a dangerous depth. In such a case it 
may have encountered a convergence in which not 
only is there no increase in density with depth, but 
also there is a downward current of water compar¬ 
able to downdrafts in the air that cause airplanes to 
lose altitude. From what is known about the shape of 
these couAergences, it seems likely that a sid:)marine 
held in one for any length of time has come into it on 
a nearly parallel course and should therefore change 
course about 90 degrees to escape. 


REST 



























Chapter 8 

LOCAL VARIABILITY 


WINDS AND WAVES 

HE great permanent wind-driven currents occur 
only in those })arts of the oceans where winds 
blow fairly constantly in one direction. Local varia¬ 
tions, particidarly in latitudes where there is a suc¬ 
cession of low- and high-pressnre areas with con¬ 
stantly shifting winds, do not lead to pronounced 
transport. When, as frecjuently happens, permanent 
currents traverse such areas, it is only by virtue of 
their hydraidic head. 

Local winds do not appear to be very effective in 
producing convergences and divergences, even on a 
small scale. In the open ocean the pattern of winds 
around a moving low-pressure area would, in theory, 
cause divergence, that is, a lessening of the thickness 
of the warm surface layer near the center of the wind 
system. On the other hand, as a low-pressure area 
passes, the winds usually increase in strength. Per¬ 
haps the tAvo effects cancel. At any rate in looking 
over the summer bathythermograms, one does not 
gain the impression that gradual variations in the 
thickness of the wind-stirred layer are produced by 
passing wind systems. 

A much more obvious effect of the rvinds is to 
transform horizontal temperature gradients into 
A'ertical gradients. Because the temperature of the 
wind-stirred surface layer increases giadually from 
north to south in the Northern Hemisphere, a south¬ 
erly Avind Avill often cause Avarmer Avater to overlie 
colder Avater. The opposite is not so often true, unless 
the horizontal gradient is Aery sharp, for a north 
Avind is usually a relatively cold Avind and therefore 
effective in maintaining a deep mixed layer. In any 
case, unless there is also a horizontal salinity gra¬ 
dient, a north Avind Avould tend to produce an un¬ 
stable situation. 

Of much greater practical importance is the eflcct 
of local Avinds on AvaAes and vertical mixing. Areas 
Avhere Avinds are variable are also variable from the 
standpoint of echo ranging. During occasional vio¬ 
lent storms, echo ranges are reduced by heavy seas. 
There is also more calm Aveather, particularly in 
summer, so that “afternoon effect” and shalloAV 
mixed layers are common. Thus on the average. 


sound conditions are poorer in the mid-latitudes 
than in regions such as the trades Avhere there are 
steady, moderate Avinds. On the other hand they are 
generally better than in the sid^polar regions where 
Avinds of gale force interfere Avith echo ranging most 
of the time. 

In its total aspect the state of the sea depends on 
many factors: on the Aelocity of the Avind, the length 
of time it has been bloAving, and the fetch (the dis¬ 
tance upAvind that the speed and direction remain 
roughly the same); on Avhether the Avind is rising or 
falling; on the temperature of the air and the differ¬ 
ence in temperature betAveen the air and the Avater. 
HoAvever, of all these, the velocity of the Avind is by 
far the most important. The aAerage seasonal and 
geographic Aariations of the Avind force are Avell 
knoAvn, and climatic atlases summarize the informa¬ 
tion in a more or less convenient form. Figures 1 and 
2 shoAv the fretjuency of Avinds stronger than force 6 
(Beaufort) in Avinter and in summer. The incidence 
of strong Avinds, as Avell as the temperature gradient 
near the surface, is considered in preparing the Peri¬ 
scope-Depth Range Sonar Charts. 

82 EDDIES 

Currents throAv off eddies Avhich vary in size from 
a feAV miles to perhaps 75 miles in diameter, large 
ones being common on either side of the Gulf Stream 
betAveen Cape Hatteras and the Grand Banks. North 
of the current the eddies contain relatively Avarm 
Avater near the surface and are easily detected. Those 
to the south of the Gulf Stream contain Avater Avhich 
is only slightly Avarmer than that already in the in- 
Aaded area, but the larger of these eddies can be 
made out from the trend of the deeper isotherms. 
From the acoustical standpoint the edges of eddies 
are much the same as the edge of the current itself. A 
shalloAv layer of one temperature Avill be found oAer- 
lying a layer of some other temperature. 

Hoav long such eddies persist and retain their ther¬ 
mal characteristics near the surface is not Avell 
knoAvn. Large eddies near the Gulf Stream are knoAvn 
to haAe persisted for more than a month, but the sur- 



72 


--— 

RESTRICTED 








FicuRt 1. World chart showing percentage frequency of winds greater than force 6 fBeauforO in February. 














































































































































IE--?**' 


t 




r**i ' r' ♦ 


/’ 

t 



f 





Figure 2. World chart showing percentage frequency of winds greater than force 6 (Beaufort) in August. 



















































































































































INTERNAL WAVES 


73 


DEGREES FAHRENHEIT 



Figurf, 3. Bath) thenuograins illustiatiiig internal waves. 


lace velocities when they first form may be as much 
as 2 knots. Presumably smaller eddies have much less 
momentum and soon die dotvu, or at any rate their 
surface characteristics are soon destroyed by wind 
stirring. However this may be, eddies are a potential 
source of poor and \ ariable sound conditions. 

Eddies have seldom been fully delineated. Usually 
a single line of temperature observations crosses what 
might be a roughly circtilar core of warmer water or 
a tongue of warmer water extending back to the 
source. 

83 INTERNAL WAVES^ 

The wa^•es hitherto considered have been displace¬ 
ments of the surface of the sea. However, waves can 
occur not only in the air-water boundary but also in 
the interface between strata of water of different den¬ 
sity. These internal waves, as they are called, have 
been observed both in the main thermocline and in 
the seasonal thermocline. Their effect on subsurface 
warfare has not yet been fully investigated, but there 
is no doubt that they are responsible for some of the 
observed variations in sonar performance. 

The difference in density above and below a ther¬ 
mocline is of course much le.ss than that between the 
air and water at the surface of the ocean. Thus a 
boundary within the ocean itself can be much more 
easily displaced than the surface of the sea, and the 
waves in the boundary set tip with correspondingly 
less energy. Theoretical considerations not only in- 

» A inemoranduni by C’.. \\’. Ufford of TC1I)\VR, dated May 
1.5, 1945, summarized work, on internal waves and provided most 
of the information in this section. 


dicate that such waves do exist but also predict their 
properties. The waves have their greatest height at 
the interface between the two layers and their am¬ 
plitude diminishes rapidly above and below it, ap¬ 
proaching zero at the stirface and bottom of the 
ocean. The theory shows further that internal waves 
can be formed not only at a boundary where the 
density changes abruptly but also in water in which 
the change is more gradual. On the other hand, there 
are no internal wa\es in homogeneous water in 
which the density does not change with depth. 

That internal waves are present in the ocean in 
fact as well as in theory has been proved by measuring 
the variation with time of the temperature, salinity 
and oxygen content at various depths. Before the 
war, measurements were made in a series with sample 
bottles and reversing thermometers from an an¬ 
chored or slowly drifting ship. As each lowering 
required an appreciable time, there was a time inter¬ 
val of more than 1 hour and sometimes as much as 2 
hours between successive lowerings. This meant that 
only long period waves cotdd be found by these 
methods. 

In this way internal waves were found with periods 
of 24 and 12 hours, corresponding to the periods of 
the tides, and with heights as great as 300 feet. There 
was some evidence of waA es with shorter periods, btit 
the periods found by this method had to be at least 
as large as the interval of 1 to 2 hours between siic- 
cessi\ e lowerings. 

In order to find out whether there were any waves 
of shorter period it was necessary to make lowerings 
in much more rapid succession. For this purpose the 
bathytliermograpJi [BT] has provided a greatly 
superior instrument. Series of lowerings every two 
minutes for as long as 24 hours have been made. A 
fetv bathythermograms from such a series are repro¬ 
duced in Figure 3 to show the kind of variations that 
are obtained. 

It is convenient to represent these waves graphi¬ 
cally as a depth curve along which the temperature 
is constant, plotted against time. In an ocean with no 
internal waves, these lines would be straight and 
horizontal; but owing to the irregular raising and 
lowering of the layers by the internal waves, the lines 
are mo\ed tip or dotvn until they represent the form 
of the waves. One of these 24-hour series, shown in 
Figure 4.\ without the small-scale features, is charac¬ 
teristic of all the records that have been obtained off 
.San Diego. The tidal period is evident in the long 


RESTRICTED 











































































































































74 


LOCAL VAKIAIULITV 


curve rc])rcscniing the tidal period. II, in Idgure 4B, 
the curve ol' tidal period is sid)tracted Iroin the short 
period curve, the resulting waves are as shown in 
Figure 4Ci. 

In addition to the lowering of bathythennographs 
from surface ships, a series ol ohser\ations made 
aboard a sid)marine showed that while balanced on 
a seasonal thermocline its depth varied periodically, 
re\ealing the existence of internal waves with periods 
of 8 to 10 minutes and heights of about 20 feet. I’hese 
observations are described in more detail in the sum¬ 
mary \ (dume on di\ ing control. On another occasion, 
in experiments off San Diego, a sulmierged sul)ma- 
rine ran through internal wa\es, maintaining a con¬ 
stant dejnh with its planes, and recorded periodic 
\ariations in the temperature at that depth. Oalcula- 
tions based on these observations indicated that the 
period of the waves was about 5 minutes and the 
largest height 28 feet. 

Experiments have also been made to find the 
velocity and length of internal waves. In order to do 
this it is necessary to make BT lowerings simultane¬ 
ously from at least three positions on the same ship 
or from three ships anchored in a triangle. "Fhe re- 
sidts of measurements made thus far suggest that the 
waves travel with a sj:)eed of about half a knot and 
have wave lengths of about 250 yards. Figure 5 shows 
a set of observations made aljoard a single ship with 
B r installations at the Ijow, amidships, and at the 
stern. Fhe ordei' in which the curves on the figure 
jirogress indicates that the waves came from the di¬ 
rection in which the shij) was heading and sticces- 
sively passed under the three installations. 

I'he practical conse([uences of internal waves in 


LOCAL TIME 

1510 1520 1530 1540 



FuaiRi. 5. I’loi'iession ol iiUCM ii;il \\a\ t's demonstrated l)y siinultaneons hatlix thei ino”i apli low eiin^s at tliree posi¬ 
tions on a sliip. 




Fk.crI'. 1. Intermd waves illnstrated by plotting the depth 
ot an isotlierm against time. 


sweep of the curve. Whives of shorter period are super¬ 
imposed upon the waves of long tidal period. The 
first section of the curve of Figure 4A is reproduced 
on a larger scale in Figure 4B, which shows the 
shorter period waves oscillating around the average 














MICROSTRUCTURE 


75 


submarine ojierations are discussed in the summary 
\olume on diving. The very important acoustical 
consequences have only gradtially and t[uite recently 
become apparent. 

"With the passage of each internal wave the depth 
of the nearly isothermal surface layer \’aries. Because 
of the changes both in layer depth and in the strength 
of the underlying negati\e gradient, the assured 
range can be expected to Aary. Thus internal waves 
are one cause of \ariability in maximum ranges, al¬ 
though not a partictdarly serious one. It woidd not 
be surprising, for example, to find that the range on 
a submarine belotv layer depth varied 200 or 300 
yards from one attack to the next Itecause of the 
effect of an internal wave, but variations much 
greater than this would be fairly tmeommon. Thus, 
o\ er a period of several hours, the variations in maxi¬ 
mum range caused by oscillations of layer depth 
wotdtl be no greater than those produced by other 
variables such as submarine aspect. Occasionally as 
the crests or troughs of the longer period (12 hours or 
so) waves passed, the sound conditions might change 
significantly, but under operating conditions hori¬ 
zontal temperature gradients coidd have as great an 
effect in any such period of time. 

It seems probaltle, however, that internal tvaves 
affect sound transmission in still another way. 

Every sound operator and antisulnnarine warfare 
officer is well aware of the fact that there is a great 
\ariability in the quality of submarine contacts 
which cannot readily be explained fjy the routine 
BT classification. One attack may be spoiled by lack 
of information due to the intermittent and mushy 
quality of the echoes, and the next attack, under ap¬ 
parently identical conditions, may be a good one. 
Idieory suggests that internal waves may be at least 
partly responsible for such variability. The simple 
refraction theory is based on the assumption that the 
temperature strata are essentially horizontal out to 
the limit of sound beam penetration. This assump¬ 
tion is obviously not correct when internal Avaves are 
present. 

As the sound Ijeam travels nearly horizontally at or 
near layer depth, there Avill be regions behind each 
AvaA’e in the surface of the thermocline where little 
or no sound strikes the thermocline, since it has been 
lient doAvn by striking the slope of the Avave facing 
the projector. I’his means that sound is removed 
from parts of the sound field Avhere it would appear 
if the thermocline Avere entirely level. On the other 


hand, internal Avaves could in some cases account for 
the penetration of sound into areas Avhere the simple 
relraction theory Avoidd ])redict a shadoAV zone. As 
the internal AvaAes progress all these effects Avill con- 
triijute to the a ariability that is one of the most strik¬ 
ing characteristics of the sound field. 

84 MICROSTRUCTURE 

Attempts to measure very small-scale thermal 
structure in the open ocean are beset with very great 
difficulties. The rolling and pitching of the ship, and 
the SAvinging of the thermometer on its supporting- 
cable are only a few of the obstacles Avhich have to be 
overcome in order to distinguish between vertical 
and horizontal gradients. NeAertheless, there is a 
good deal of more or less indirect evidence that small 
but sharp temperature gradients exist in the Avater 
Avhich may serve to scatter sound. 

In the first place, even the BT, crude as it is for 
studying small-scale thermal structure, occasionally 
shoAvs iq) surprising detail, Avhich is reproduced in 
both the up and the doAvn traces, and is therefore 
vertical structure. Figure 6 is a magnified rej)roduc- 
tion of BT traces taken in Guantanamo Bay, Cid^a, 
Avhich shoAV pronotinced development of successive 
strata of water, each Avith a relatiAcly uniform tem¬ 
perature and separated from the next by a sharp dis¬ 
continuity. Idle loAverings Avere made sloAvly on a 
hand-line paid out from a dinghy in order to record 
the temperature with the greatest possible accuracy 
and freedom from vibration. In each figure the doAvu 
trace is on the right and the iqi trace on the left. 
Failure of the traces to coincide is due to lag in the 
thermal response of the instrument tised and to the 
fact that it Avas haided in more quickly than it Avas 
paid out, Avhich tended to smooth the temperature 
traces. Nevertheless, there is sufficient agreement in 
each pair of traces to leaAe little doubt that valid 
measurements of vertical structtire Avere olitained. 

Measurements of vertical sound Aclocity gradients- 
reveal small-scale variations Avhich are jiresumably 
largely due to temjierature changes and therefore are 
indicative of thermal microstructure. The observa¬ 
tional method used did not preclude the possibility 
that part of the recorded Aariations Avere caused by 
A'ertical oscillations of the instrument through the 
Avater or by horizontal gradients, but comparison of 
doAvn and iqi traces shows that some vertical micro- 
structure existed. HoAvever, the interest in this Avork 








76 


LOCAL VARIABILITY 


DEGREES FAHRENHEIT DEGREES FAHRENHEIT 



Figure 6. liathythermograms illustrating microstructure. 


docs not lie in its accuracy as a means of studying 
microstriicture but rather in the fact that it is a con¬ 
crete demonstration of the existence of small scale 
variations in velocity within the sound field which 
can be in part responsible for variability in sound 
transmission. 

Attempts to measure small-scale horizontal struc¬ 
ture have not been very successful, although it is 
fairly clear that near the surface convection cells, 
when they are well de\eloped, must cause just such 
thermal structure. The temperature increase has 
been measured^ between depths of 3 and 20 milli¬ 
meters and has been found to be as much as 1.8 F, 
averaging about 0.8 F. 'I bis gives some idea of the 
maximum degree of thermal instability that can arise 
through surface cooling and of the maximum 
amount of horizontal ^•ariation that might be ex¬ 
pected when the surface water drains into the con¬ 
vergences of convection cells. Thus far no accurate 
measurements have been made of the temperature 
differences within these convergences, although in 
horizontal temperature measurements at various 


depths some er idence has been reported-^ of small- 
scale cyclical rariations of aliout 0.02 degree occur¬ 
ring about every ten yards in waters that were in¬ 
vestigated off San Diego, as well as occasional larger 
discontinuities of the order of 0.5 degree. 

The acoustic effect of microstructure has not yet 
been measured accurately, l)ut (jualitatively it is 
fairly obvious how it affects sound transmission. A 
steplike thermocline of the kind pictured in Figure 
6 is in reality a series of little thermoclines of varying 
degrees of steepness. Moreover, they are not horizon¬ 
tally continuous throughout the entire sound field 
because every BT lowering shows a slightly different 
pattern. I'he combined effect is that the individual 
refraction paths of sound rays are highly variable, 
leading to irregular variations in intensity within the 
sound field. Proliably microstructure is one of the 
more important causes of varying intensity from one 
echo to the next, as contrasted with the internal 
waves previously discussed, which would be more 
likely to produce larger scale fluctuations of a more 
regular nature over a period of several pings. 


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Chapter 9 

COASTAL WATERS 


N ear most continental coast lines the vertical 
stnictnre of the rvater column is so different from 
that jjrevailino in the open ocean as to warrant sep¬ 
arate discussion, d liis is especially true of coasts hav¬ 
ing a broad continental shelf, where large or many 
rivers drain into the sea, or where offshore winds 
prevail. In general, the situation in coastal waters 
tends to be sjjecialized through one or more of these 
causes. Vertical gradients of temperature and salinity 
are often much more pronounced than offshore, and 
the seasonal changes are nsnally greater also. The 
100-fathom contour roughly marks the boundary be¬ 
tween coastal and offshore water, although in special 
circumstances water which is clearly of coastal origin 
can be encountered 100 miles or more beyond the 
limits of the continental shelf. The currents in 
coastal waters, which are caused by density gradients 
and by local winds and tides, are complex and inter¬ 
esting oceanographically and highly important from 
a practical standpoint l)oth in navigation and in 
echo ranging, dlie type of bottom also is influential 
in determining whether sound ranges will be im¬ 
proved by multijile reflections or will be shortened 
bv masking reverberation. 

J 

9 1 COASTAL CIRCULATION 

9 ' i Density Currents 

Because of land drainage the density of coastal 
water is usually less than at corresponding depths off¬ 
shore. Along any coast where there is a significant 
amount of freshening due to land drainage, there is 
a general tendency for the fresh water to move ofl- 
shore on the surface and the denser (more saline) 
water to move in underneath. Thus the salinity of the 
surface water increases from the shore outwards, and 
furthermore there is an increase in salinity from the 
surface downward. This is an unstable condition in 
that the density surfaces slope downward in an in¬ 
shore direction, and the unecpial pressure gives rise 
to currents which in general behave according to the 
principles of the circidation theory. 

Since the density surfaces slope downward toward 
the shore line, the direction of the currents is roughly 


parallel to the coast, with the coast on the right-hand 
side of the direction of flow in the Northern Hemi¬ 
sphere and on the left-hand side in tlve Southern 
Hemisphere. 

As a result of this prevailing situation, in the 
Nortliern Hemisphere the coastal currents on the 
west side of the oceans are usually south-flowing and 
therefore relatively cold as shown in Figure 1, which 
is a temperature section across the Labrador Current. 
I'he observations were made in spring, and seasonal 
warming had affected the surface water. However, 
this did not obscure the fact that the inshore water 

DISTANCE IN MILES 


0 100 200 



Fk;ure 1. Temperature profile across the Labrador 
Current. 

DISTANCE IN MILES 


0 lOO 200 



RESTRICIED 


77 














78 


COASTAL WATERS 


that had come down from the north was much colder 
than the water further offshore. Figure 2 is a salinity 
section of the same water. The salinity is least near 
shore and at the surface. Salinity is responsible for 
the density gradient that sets uj) the current, since 
from the standpoint of temperature alone, the in¬ 
shore water obviously would be heavier rather than 
lighter. 

Along the east side of the oceans, coastal density 
currents generally flow northward and therefore are 
warmer than the offshore water. Thus both tempera¬ 
ture and salinity contribute to the density gradient 
between inshore and olfshore water. 

Exceptions to these general statements occur where 
the continental shelf is narrow and where little river 
water is brought down to the sea. In such cases local 
winds and seasonal variations in rainfall are im¬ 
portant in determining whether or not a typical 
coastal current will develop. For example, on the 
east coast of the United States there is a well-devel¬ 
oped coastal current as far south as Fort Pierce, 
Florida. Beyond that point the shelf narrows rapidly 
and the coastal current, known as the Florida Coun¬ 
tercurrent, becomes weak and sometimes very nearly 
nonexistent. The coastal water off Miami is slightly 
cooler than the northward flowing Florida Current 
offshore (see Figure 19, Chapter 7). It is also slightly 
fresher, enough so that the countercurrent generally 
flows during the wetter months of the year. During 
the spring and summer months, however, steady 
southeasterly breezes carry the Florida Current close 
inshore, and the coastal current is either obliterated 
entirely or is driven below the surface where it can 
be detected only by the BT or by other subsurface 
sampling methods. 

A somewhat similar situation occurs off the Cali¬ 
fornia coast. Figure 3 shows the oceanic California 
Current flowing well offshore and the coastal current 
known as the California Countercurrent which flows 
northward closer inshore. The latter is typical in so 
far as it is a warm, north-flowing current, bin the 
temperature difference is slight, and there is not 
enough coastal drainage to produce a significant 
salinity gradient. Flence it is weak and diffuse, fre- 
(jiiently permitting the development of still another 
countercurrent close inshore, as shown in the figure. 
During the summer, when the ])re\ailing winds are 
offshore, the coastal current occurs only in the sid)- 
surface ■waters and is rejilaced at the surface by a 
complex circulation resulting from upwelling. 


d he extent to which a current parallels a coast 
depends to a considerable extent on the difference in 
density between inshore and offshore water. When 
the density difference is slight, the current may Ire- 
(piently be set onshore or offshore. 1 he directions 
that it takes will be depeiulent to a large degree on 
bottom topography. Frictional retardation by bot¬ 
tom particles in contact with a current not only 
affect its velocity but also its direction. The tendency 
is for the normal twisting to the right (in the North¬ 
ern Hemisphere) to be accentuated as the current 
moves o\er a shoaling bottom and to be lessened over 
a deepening bottom, d’hus when the water is rela¬ 
tively homogeneous it is common to find an inshore 
set towards reefs and inlets (Figure 4). 

- Local Wind and Tidal Currents 

A\4th an onshore wind, surface water is transported 
toward the land, with the result that the surface layer 
thickens near the beach. A\4th an olfshore wind the 
warm surface layer is removed and the underlying 
thermocline apjjroaches the surface. 1 his process is 
called up^velling, although the term is somewhat 
misleading, as little vertical motion is involved. 

\Vhere steady offshore Avinds prevail, as in the 
trade Avind belt, upAvelling may be a more or less 
continuous phenomenon. The mixed surface layer is 
carried offshore by the Avind as fast as it is formed. 
Fxam|ffes are the Avest coast of Africa in the region 
of Dakar and the Gulf of Panama. 44) a lesser extent 
similar conditions occur on the lee side of islands in 
the trade Avind belt. 

Along the east coast in higher latitudes, upAvelling 
may occur as a residt of strong offshore gales in 
Avinter. Several oceanographic stations made in Feb- 
rnaiy on a line across the Nova Scotian continental 
shelf shoAV a mi.xed surface layer only al)out 100 feet 
deep overlying much Avarmer and more saline Avater. 
File strong positive gradients thus produced Avould 
give a deep sidimarine almost complete jirotection 
against sonar detection. 

While no other Avinter temperature observations 
are available from this area, it is lielieved that nor¬ 
mally a much deeper isothermal surface layer Avould 
be found Avith positive gradients confined to the 
outer third of the continental shelf. At any rate that 
is the jirevailing situation further south. A jiossible 
explanation of these relatively shalloAV and sharji 
positive gradients off Halifax is that strong offshore 







COASTAL CIRCULATION 


79 




winds had recently removed much of the superficial 
layer from the continental shelf and that this water 
had been replaced by a deep indraft of much ^varmer 
and more saline oceanic water. 

Apparently on a broad continental shelf, as off 
Halifax, bottom friction and the strong southward 
flowing coastal current are usually able to combine 
in preventing all of the coastal water from being 
blown out to sea by the prevailing offshore winds. 
Nevertheless, a certain amount of upwelling must 
obviously occur. \Vhere the continental shelf is nar¬ 
row and the coastal current is weak because of lack 


of land drainage, offshore -winds will be much more 
effective in decreasing the depth of the mixed surface 
layer close to the coast and thus reducing the effec¬ 
tive echo range. 

The converse to upwelling, the accumulation of 
warm surface water near a coast by an onshore tvind, 
improves sound conditions and is equally common. 
It is a special case of convergence, just as upwelling is 
a special case of divergence. 

Tides are a form of wave motion, and as such their 
bcha\ ior is in many respects similar to that of wind- 
driven waves. In the open ocean where the height of 





























80 


COASTAL WATERS 



FiciiRi; -}. Configuration of surface currents, Xewfouucl- 

laiul area. 

the wave is less than 3 feet and its length several hun¬ 
dred miles the orbital movement of water particles 
in the wave extends to the bottom. The mass transfer 
of water inxolved in this movement is large, but the 
horizontal displacement of individual particles is 
small. However, as the wave approaches shoaling 
water the horizontal components of flow are greatly 
increased because of the decreasing cross sectional 
area. The resulting tidal current is accompanied by 
a shortening of the wave length and an increase in 
the height of the tide. 

In a restricted area such as a channel, tidal currents 
generally flow back and forth, reversing direction 
with each tidal period. In more open shallow waters 
tidal currents have a rotary motion which is appar¬ 
ently an effect of the earth’s rotation. I'he direction 


shifts around the compass during each tidal period 
(generally every 12 hours), and any given water par¬ 
ticle in the tidal stream describes a roughly elliptical 
course. 

Georges Bank olf the Gulf of Maine jmovides a 
notable example of the ellects of strong tidal cur¬ 
rents. Immediately north of this bank in the deeper 
waters of the Gulf of Maine there is a pronounced 
seasonal cycle with well-established vertical stability 
(Figure 17). I'o the south of it is a typical sloj)e water 
area (Figure 40, Ghapter 5). But inside the 5()-fathom 
contour on the bank itself stability develops only 
Ijriefly and intermittently dining a short summer 
season whenever the winds happen to be light for sev¬ 
eral days at a time. At all other times tidal stirring 
and winds keep the water column mixed from surface 
to bottom. As a conseipience relatively low surface 
temperatures are encountered over the bank in sum¬ 
mer. This of course explains the widespread reputa¬ 
tion of Georges Bank for fog. Another result of the 
strong tidal stirring is that near the edges of the bank 
rather sjiecial hydrological conditions are found 
tvhere the mixed waters of the bank are in contact 
with the stable waters surrounding it. This is illus¬ 
trated in Figure 5 which shows the temperature 
across Georges Bank in northwest-southeast profile 
in June 1939. Not only are the sound conditions, 
provided reverberation is not limiting, markedly 
better over the bank than on either side, but also a 
submarine crossing such an area would find that trim 
for resubmergence would alter significantly. 

Tides and variable winds also affect the speed and 
direction of coastal currents. As their \elocity varies, 


DISTANCE IN MILES 

0 50 100 150 200 



Figure 5. Temperature profile across (ieorges Bank. 


RESTRICTED 


ill 
























COASTAL CIRCULATION 


81 


more or less crosscurrent transfer takes place. This is 
usually offshore at the surface and inshore along the 
bottom. It is the inshore component near the bottom 
Avhich is especially significant, for it is this ^vhich is 
chiefly responsible for the large vertical salinity gra¬ 
dients typical of coastal waters. Especially where 
deep gidlies cross a continental shelf, highly saline 
water will be able to move in close to the land, while 
abo^e it much fresher surface water will be moving 


along shore, occasionally with a slight offshore com¬ 
ponent. It is important to remember that if coastal 
craters are not to become fresher and fresher, there 
must be an inward movement of saline water along 
the bottom, which through vertical turbulence car¬ 
ries salt up to the surface. The stronger tidal cur¬ 
rents near the coast are favorable for such vertical 
mixing, but the high thermal stability typical of the 
summer season has the opposite influence. 


PETIT MANAN- 
MT. DESERT ROCK- 
MATINICUS ROCK" 
SEGUIN ISLAND"" 
BOON ISLAND"' 
GLOUCESTER^ 


POLLOCK RIP 
NANTUCKET 
VINEYARD SOUND 
BLOCK ISLAND 



FIRE ISLAND-- 

SANDY HOOK-- 

FIVE FATHOM BANK-^— 


WINTER QUARTER 


BODIE’S ISLAND- 

CAPE LOOKOUT- 

FRYING PAN SHOAL- 

RATTLESNAKE SHOAL- 

MARTINS INDUSTRY- 


FOWEY ROCKS 
CARYSFORT REEF 


DRY TORTUGAS- 



CD 

cr 

_j 

>- 

LlI 

>- 

o 

h- 

1— 

> 

o 

UJ 

< 

(T 

CL 

< 

z 

_1 

3 

CL 

o 

o 

UJ 

U- 



Z) 

—> 

3 

< 

LJ 

0) 

o 

z 

o 


Fk;lre 0. Seasonal teinperatiue cycle in shallow water oft the eastern coast of the United States. 




RE.STRICTEI' 

























































82 


COASTAL WATERS 


From the foregoing discussion, tlierefore, it is aj)- 
parent that tidal currents have a direct effect on all 
phases of coastal stdmiarine warfare in the following 
ways: first, they tend to destroy \ertical temperature 
and salinity gradients, and second, in many localized 
areas they produce mixed water where otherwise it 
would not exist. This naturally im])ro\'es echo rang¬ 
ing conditions. It also simplifies diving operations of 
sidDutarines but at the same time makes evasion more 
difficidt. 


92 VERTICAL DISTRIBUTION OF 
TEMPERATURE AND SALINITY 

Excejit in a few sj^ecialized areas in low latitudes, 
the vertical temperature gradients in coastal waters 
are dominated by the seasonal cycle. In general, 
coastal waters are sufficiently shallow so that in 
winter the whole water column becomes isothermal. 
Thus there is no counterpart to the main thermo- 
cline of the open ocean. 



Figure 7. Surface minus imltom temperature, July and .\ugust: (tulf of Maine to Chesapeake Bay. 


I RIC I ED 


0 













VERTICAL DISTRIBUTION OF TEMPERATURE AND SALINITY 


83 


I he a\ailable BT observations permit illustrating 
the seasonal temperature cycle in most detail lor a 
large area along the eastern coast of the North Atlan¬ 
tic. I'he many oceanographic stations which are 
available Irom the western North Pacific indicate 
that the coastal conditions there are entirely com¬ 
parable. 

Cdose to the beach the relati^'ely strong tidal cur¬ 
rents and the shallowness of the water usually pre- 
\ent marked thermal stability from developing out 


to roughly the lO-fathom contour, ^\’’ith this in mind, 
a useful sort of diagram has been developed from the 
routine surface temperatures collected from light¬ 
ships along the Atlantic Coast (Figure 6). It gives a 
fairly representative picture of the seasonal tempera¬ 
ture changes at the surface as a function of latitude 
and although these are inshore surface temperatures, 
they are on the average only slightly lower at any 
given latitude than the surface temperatures across 
the whole continental shelf. 



RESTRICTED 















8-4 


COASTAL WATERS 


DISTANCE IN MILES 



Figi rf- 9. Temperature profile SE from Moiitauk I’oint in midwinter. 


Detailed studies have been made of the distribu¬ 
tion of temperature and salinity in the Gulf of Maine 
and southward o\er the continental shelf to Cape 
Hatteras. For the present purposes, the significant 
conditions over this area can be summarized in two 
diagrams: the first (Figure 7) shows the average differ¬ 
ence in temperature between the surface and the bot¬ 
tom in July and August, while the second (Figure 8) 
shows the magnitude of the vertical salinity gradient 
at the same season. Thus it will be seen that in sum¬ 
mer o^•er much of this area surface temperatures are 
from 20 to 30 degrees higher than at the bottom, 
while the surface waters are from 1 “/oo to 3.5 ^/oo 


fresher than at the bottom. The resulting vertical 
change in the speed of sound is therefore generally 
greater than 150 feet per second and is in some areas 
as great as 200 feet per second. 

Profiles of temperature and salinity constructed 
from observations made at different seasons on a line 
extending southeastward from Afontauk Point, Long 
Island, serve to illustrate Ijoth the seasonal c)cle and 
the horizontal changes in the offshore direction. 

Starting in midwinter the water column is virtu¬ 
ally mixed from surface to bottom across most of the 
continental shelf (Figures 9 and 10). The tempera¬ 
ture and salinity, however, both increase gradually in 


DISTANCE IN MILES 



r 


RESTRICTED 













































VERTICAL DISTRIBUTION OF TEMPERATURE AND SALINITY 


85 


DISTANCE IN MILES 

0 100 200 300 400 



the offshore direction, and the isotherms and iso¬ 
halines are on the whole quite parallel, the warmer 
water being the more saline. It is typical that near the 
edge of the continental shelf this increase is some¬ 
what more rapid near the bottom, corresponding to 
the inshore component of the bottom water. At this 
season the crosscurrent density gradient, and there¬ 
fore the strength of the current, is at a minimum, cor¬ 


responding to the decreased land drainage of the 
winter months when much of the precipitation re¬ 
mains on the land in the form of snow and ice. 

The spring conditions are illustrated by Figures 11 
and 12. As the surface waters stabilize, because of 
vernal warming, surface salinities decrease, both be¬ 
cause of decreased vertical turbulence and the spring 
freshets. Thus over the continental shelf the warmer 


DISTANCE IN MILES 



RESTRICTED 




















COASTAL WATERS 


86 


100 


DISTANCE IN MILES 

200 300 


400 



water becomes the less saline, which is the reverse of 
the situation in midwinter, 

V'ertical stability increases and consetjiiently \er- 
tical mixing- decreases until midsummer (Figures 13 
and 14). Under such conditions lateral mixing is in¬ 
creased in the layer of greatest stability. This may 
partly explain the characteristic outward bidge of 
the isotherms at mid-depths over the continental 
slope. Here the isotherms and isohalines cross each 


other in a complex manner which is not well under¬ 
stood, but which is also at least partly caused by the 
strongest current, in this case southwestward, being 
concentrated near the 100-fathom contour. Thus 
near the offshore end of the coastal water section, 
cold water from the north is being carried into the 
profile more rapidly than further inshore. 

During the autumn the coastal water, because of 
the j^revailing cold and offshore winds, cools more 


DISTANCE IN MILES 

0 100 200 300 400 


























VERTICAL DISTRIBUTION OF TEMPERATURE AND SALINITY 


87 


DISTANCE IN MILES 



rapidly than the offshore water. Thus positive gra¬ 
dients, both of temperature and of salinity, develop 
over the continental slope (Figures 15 and 16). 

The major features described in these sections off 
Montauk Point could be made out in any profde 
across the continental shelf from Labrador to 
Florida. Except off Cape Ffatteras and off southern 
Florida, along the whole of this coast the continental 
shelf is broad and well-developed. Fresh water is 


brought down to the sea by many rather evenly 
spaced rivers. Consequently a southward-flowing cur¬ 
rent, which tends to be strongest along the 100- 
fathom contour, follows the continental shelf south¬ 
ward and is only partially interriq^ted by such major 
irregularities as the Laurentian and Fundian 
Channels. 

In Figures 17-19 the seasonal cycle at various points 
along this coastline is illustrated in the same manner 


DISTANCE IN MILES 



COASTAL 

WATER 


—SLOPE WATER 


_GULF_ 

STREAM 


SARGASSO SEA 


Figure 16. Salinity profile SE from Montauk Point in autumn. 


RESTRICTED 













































DEPTH IN FEET DEPTH IN FEET DEPTH IN FEET 


88 


COASTAL WATERS 





Figure 19. Average monthly temperature-depth curves in coastal waters between New York and Cape Hatteras. 


RESTRICIED 































































VERTICAL DISTRIBUTION OF TEMPERATURE AND SALINITY 


89 


as was used in Part 2 above (Figures 34 to 42, Chap¬ 
ter 5). A comparison between the two sets of diagrams 
serves to emphasize the relatively great stability of 
coastal waters as compared to offshore waters. Only 
in shallow areas where strong tidal currents occur is 
this not generally true. 

In spite of the fact that the temperature-salinity 
correlation in coastal waters varies widely, both sea¬ 
sonally and geographically, with only a few excep¬ 
tions the vertical temperature distribution controls 
the sound refraction pattern. As in deep water, this 
results from the fact that in layers where the salinity 
is changing most rapidly with depth, temperature 


does likewise. Only when the salinity giadients are 
unusually strong is the BT lowering likely to be mis¬ 
leading from the acoustical standpoint. For example. 
Figure 20A shows the vertical distribution of tem¬ 
perature and salinity at the mouth of the Baltic Sea 
in May 1937. Here relatively fresh and warm Baltic 
water overlies colder and more saline Atlantic water, 
and the depth of the thermocline more or less coin¬ 
cides with the depth of the maximum salinity gra¬ 
dient. A BT reading would indicate sharp downward 
refraction with very short echo ranges. The salinity 
gradient, however, partly compensates for the tem¬ 
perature effect. Both together are equivalent acous- 


TEMPERATURE 


40® 60® 80® 40® 60® 80® 



LU 

UJ 

U_ 


X 

H 

CL 

UJ 

Q 


Figure 20. Acoustically critical temperature and salinity gradients in coastal waters. 


RESTRICfEDl 






































90 


COASTAL WATERS 





Figurk 21. Ratli\ thermograni.s from probable areas of critical salinity gradients. 


tically to a temperature decrease of about 1 degree in 
the upper 60 feet, and the predicted range is medium. 
I’hus it is apparent that in extreme cases salinity gra¬ 
dients can be important, but that a very large salinity 
gradient will be compensated by a relatively weak 
temperature change. In Figure 20B, which shows a 
more typical temperature-salinity distribution in 
that the salinity gradient is not so extreme, a reason¬ 
ably accurate prediction can be made from tempera¬ 
ture alone. 

Sometimes, however, strong salinity gradients oc¬ 
cur when there is little or no vertical change in tem¬ 
perature. Examjdes are shown in Figures 20C and 
20D, which represent conditions olf the mouth of the 
.Amazon. The salinity gradient in Figure 20C is 
highly significant since it is not compensated by a 
temperature change. In this case there will be sharp 
upward refraction, and it is unlikely that a vessel 
with conventional (non-tilting) sonar gear will be 


able to get contact on a submarine below the fresh 
surface layer. 

Figure 201) differs from the preceding one in that 
the salinity gradient is probably largely confined to a 
shallow surface layer above projector depth. When 
this is the case, it ob^iously will not interfere with 
echo ranging. It is difficult to say how common it is 
to find salinity gradients of this sort because oceano¬ 
graphic water samples have seldom been spaced at 
close enough depth inter\als to show the complete 
\ ertical structure. Howe\ er, there are many examples 
of bathythermograms such as the one in Figure 21.\, 
in which a very shallow positive temperature gra¬ 
dient indicates the existence of a fresh surface layer 
too thin to affect echo ranging. 

1 he remainder of Figure 21 shows other examples 
of bathythermograms from areas where strong salin- 
ity gradients are ordinarily found. Their existence is 
obvious in Figure 21B, but in C and D, taken near 












































































































































































































































































































































































































BOTTOM STRUCTURE 


91 


the Amazon in areas where it is almost certain that 
such gradients occurred, there is nothing in the 
l)athythermograms to indicate their presence. 

In summarizing the acoustic effects of salinity gra¬ 
dients, it is apparent that there are two main factors 
to be considered: (1) the depth of the gradient, since 
it is not important if it is above projector depth; (2) 
the amount of change in salinity with respect to tem¬ 
perature changes. The worst situation from the 
standpoint of echo ranging occurs when the water 
column is essentially isothermal, because then not 
only is the salinity gradient most effective but also 
the bathythermograms are most misleading. This 
condition is cpiite common in the tropics. In tem¬ 
perate and arctic regions the outflow from rivers is 
likely to be warmer than the ocean water in summer 
and colder in winter. Therefore it is only for short 
j)eriods in the spring and autumn that salinity gra¬ 
dients are likely to occur without corresponding 
temperature gradients. 

Since density currents must flow parallel to coasts, 
the water of reduced salinity which is formed near 
the mouths of rivers tends to spread along the coast 
instead of flowing out to sea. The influence of a large 
river may be noted hundreds of miles away. Thus in 
the Gulf of Mexico a fresh surface layer extends west¬ 
ward from the mouth of the Mississippi, and the 
freshening influence can be detected, particularly in 
spring, beyond Galveston (Figure 20B). In areas of 
adequate rainfall, therefore, continuous bands of 
water of low salinity are formed along the coasts, and 
the influence of the rivers seldom extends far out to 
sea except near the equator where Coriolis force is 
much reduced. 

Minor and temporary cases where the refraction is 
controlled by vertical salinity gradients can be found 
at New London, Connecticut, where the outflow 
from the Thames River sometimes overlies virtually 
isothermal water of about the same temperature 
which has been mixed by the strong tidal currents of 
the narrow entrances to Long Island Sound. 

At the edge of the continental shelf, where near 
the surface the relatively fresh coastal water tends to 
overlie more saline water, one would expect occa¬ 
sionally to find situations where vertical salinity gra¬ 
dients controlled the refraction. No pertinent obser¬ 
vations have been uncovered, but since the band 
where the right conditions prevail is narrow, the 
oceanographic stations do not usually come at just 
the critical point. 


9.3 BOTTOM STRUCTURE 

It was pointed out in the introductory statements 
that both bottom topography and the composition of 
sediments are important in echo ranging. Whether or 
not it is an advantage for an echo ranging ship to 
work over a bottom that is a good reflector of sound 
depends entirely on the direction in which the sound 
is reflected. A smooth, hard bottom that reflects 
sound on ahead is ideal, and ranges will be nearly as 
good as in deep water with perfect temperature con¬ 
ditions. But if bottom irregularities intercept sound 
at nearly right angles, reflecting it back toward the 
sound source, the result is reverberation and skip 
distances. The size of the irregularities does not need 
to be very great. Stones or ripple marks only 3 or 4 
inches high will raise the reverberation level of super¬ 
sonic gear markedly. Hence stony, rocky, and coral 
bottoms are almost always poor from the echo rang¬ 
ing standpoint because of their roughness. Sand bot¬ 
toms are usually good because they are not only good 
reflectors but are also generally smooth. Occasionally 
they may be poor, however, when there is enough cur¬ 
rent over the bottom to produce ripple marks, and 
there is some evidence that a particular sand bottom 
may vary considerably in this respect according to the 
way it is disturbed by tides and storms. 

The manner in which these generalizations fit the 
observed facts is illustrated by a recent analysis^ of all 
the observed maximum ranges and suitable acous¬ 
tical measurements in shallow water. It was found 
that for all refraction patterns over rock and coral 
bottoms, ranges were less than 1,500 yards 60 per cent 
of the time and averaged about 1,000 yards. Maxi¬ 
mum ranges over sand were less than 1,500 yards 
only 20 per cent of the time and averaged 2,500 yards, 
while over sand and mud the ranges were less than 
1,500 yards 30 per cent of the time and averaged 
about 2,200 yards. Over soft mud bottoms, results 
varied according to the refraction pattern as in deep 
water. 

The methods of correlating bottom types with 
acoustic phenomena were discussed in some detail in 
Ghapter 2, as well as the principles used in construct¬ 
ing bottom sediment charts for tactical use by naval 
forces. This work can be understood more clearly, 
however, by considering briefly how the different bot¬ 
tom types are produced in the first place and how 
various oceanographic factors affect their distribu¬ 
tion. 


RESTIUn 




92 


COASTAL WATERS 


Marine sediments as a whole are a very hetero¬ 
geneous collection of all the materials carried into 
the sea or produced in it which are heavy enough to 
sink to the bottom and inert enough so that they have 
not dissolved or decomposed into soluble substances. 
A fairly large proportion of the sediments is deri\ed 
from land. Direct erosion of the shore line by wave 
action is one of the most important .sources of such 
material. River drainage depositing its load of silt in 
the sea is also important, as is glacier movement. A 
less obvious but certainly significant source of marine 
sediments is air-borne dust derived from arid regions 
and from volcanic explosions. The sea produces some 
of its bottom sediments, for the most part by bio¬ 
logical processes (shells, coral) but also by precipita¬ 
tion of inorganic chemical substances from sea water 
and by accumulation of the products of submarine 
volcanic activity. Taken altogether these are prob¬ 
ably the most important sources of marine sediments 
although there are a great many other contributing 
agencies of minor significance. 

I'he sea is continually moving and reworking the 
loads of sediment brought into it. The physical force 
of waves and currents carries the smaller particles 
along, grinding them against each other and against 
larger stationary objects on the bottom. The eroding 
effect of such action is obvious on any rocky beach, 
where the smaller pebbles are rounded and the sur¬ 
face of the larger rocks and boulders are worn 
smooth. I'he tendency then is for boulders to be 
transformed gradually over long periods of time into 
mud. Over similarly long periods chemical activity 
is also important, dissolving and decomposing sedi¬ 
mentary materials. Some minerals are far more sid> 
ject to chemical as well as physical dissolution than 
others. The hardness and inertness of quartz, for ex¬ 
ample, is responsible for its abundance (in the form 
of sand) on the continental shelf. It remains behind 
while other less stable substances are reduced to a 
finer state and carried further out to sea. 

T ransportation of sediments obviously depends on 
both particle size and the speed of the current. Sup¬ 
pose, for e.xample, a swift-flowing river delivers into 
the sea a load of sediment ranging in particle size 
from fine mud to sand. In the slower moving ocean 
currents the sediment will settle gradually, the larger 
particles going to the bottom more cpiickly, so that 
they are not carried as far from the source as the 
smaller sizes. T hus, speaking in broad terms and neg¬ 
lecting many local variations, sediments tend to be 


coarse near shore, grading to fine mud in the deep 
oceanic basins. 

Particles too large to be carried completely in sus¬ 
pension may be lifted momentarily and carried for 
a short distance before they settle back to the bottom. 
Still larger particles may be rolled along. And finally, 
a certain amount of transportation may occur in the 
absence of currents. The familiar terrestrial phenom¬ 
enon of landslides has its counterpart on the sea floor. 
Mud slides are a fairly common phenomenon wher¬ 
ever silt settles on a sloping bottom. Because the 
buoyancy of the water makes the friction between 
the mud particles much less than it would be in an 
aerial environment, mud slides ma\ occur on very 
slight slopes. 

The type of bottom that occurs in any particular 
area depends on the interaction of the factors that 
have been mentioned—the kind of sedimentary ma¬ 
terials available, the way they have been worked over 
by marine agencies, and the way their deposition is 
affected by local currents. It is possible to tell a great 
deal about the bottom simply by examining a chart 
that shows the shore line and bottom topography, 
since these are indicative of both the character of the 
materials and the current pattern. Thus it is natural 
to find mud deposits off the mouth of a large river, or 
a smooth, sandy bottom lying off a low, sandy coast. 
Or, if the shore line is irregular, with headlands and 
estuaries, the bottom is likely also to be variable both 
as to topography and l)ottom type. It probably will 
be rocky off the headlands, in the mouths of rivers, 
and around sidmierged reefs where the currents are 
strong enough to scour the bottom and carry away 
finer sediments. On the other hand, small basins will 
collect deposits of sand or mud. 

Further offshore, topographic features of the bot¬ 
tom are associated with similarly characteristic sedi¬ 
ment types. Sid:)marine ridges or sharp changes in 
depth such as occur at the edge of the continental 
shelf are nearly always accompanied by currents that 
expose rock surfaces or at least ripple the sand suffi¬ 
ciently to make echo ranging difficult. Offshore banks 
with their strong rotary tidal currents also show pro¬ 
nounced ripple marks, often at depths of nearly 100 
fathoms. Currents can affect sid^marine topogiaphy 
as well as be affected by it, since a current strong 
enough to transport sand must ultimately deposit it 
somewhere, generally in the form of a ridge or bank 
at the current’s edge. 

In the tropics, coral reef formation is often the 


restricted”" 


i— 





UOTTOM STRUCTURE 


d3 


dominant feature of submarine topography in shal¬ 
low water, and in such areas high reverberation 
makes echo ranging very difficult. Of similar acous¬ 
tical (juality is the rough, rocky bottom in the neigh¬ 
borhood of islands produced by submarine volcanic 
eruptions. 

Bottom topography and structure have always 
been ol considerable interest from the standpoint of 
navigation, and most coastal areas have been sur- 
xeyed at one time or another with varying degrees of 
accuracy and completeness. I'he depth of water has 
been measured by sounding leads and more recently 
by acoustic fathometers which provide far more com¬ 
plete data. Bottom type was largely determined from 
small samples collected by a cup or tube on the end 
of the sounding lead or by grease smeared in a de¬ 
pression on its lower end. 

Oceanographic expeditions have also obtained a 
considerable amount of information about bottom 
type and topography. In general, larger samples of 
bottom have been collected than in the case of na\ i- 
gational surveys. Scientific description of bottom 
structure retpiires large samples. Since often the bot¬ 
tom is composed of widely different sizes and kinds of 
fragments, a small sample can be very misleading. 
For this reason oceanographers have used various 
types of dredges, tubes, and containers with jaws that 
snap shut on contact with the bottom. More recently 
the underwater camera referred to in a jnevious 
chapter has been used extensively in such studies. It 
takes a picture of about 50 square feet of the bottom 
and shows many details of bottom structure and 
small-scale topography that are valuable supple¬ 
ments to the information obtained by sampling de¬ 
vices. Examples of bottom pictures are shown in 
Figures 22-24. 

Charts of bottom topograph) can be prepared in 
very great detail by using modern recording fathom¬ 
eters. It is a far more laborious task to sample the 
bottom adequately for charts of sediment types. This 
has been done in a few local areas for scientific pur- 
jK)ses and in connection with the acoustical tests de¬ 
scribed in Chapter 2. In other places, where only 
goxernment surveys have been made, the bottom no¬ 
tations on the charts are not completely adequate for 
acoustical purposes. \Vhere soundings are not closely 
spaced, it is necessar)' to decide whether or not a par¬ 
ticular area is divided into patches or is continuous, 
and what its areal extent and probable boundaries 
may be. I'hus it is necessary to depend a great deal 



Figi re 22. Photograph of a .smooth, sandy hottom. 


on general oceanographic and geological knowledge 
about the kind of sedimentary materials available in 
the region and how they might be deposited accord¬ 
ing to local currents and bottom topography. 

In order to understand more fully just what the 
retjuirements are for a bottom sediment chart, the 
chapter ends with a discussion of the material col¬ 
lected by the government surveys and how it has been 
used for purposes of subsurface warfare. 

For the coutiuental United States and its posses¬ 
sions, which have been surveyed by the Coast and 
Geodetic Survey, the data are unusually complete. In 
addition to the bottom notations on the printed 
charts, a great many of the samples have been pre¬ 
served, and access may be had to the original field 
data sheets. These sheets are drawn on a much larger 
scale and contain many more soundings and bottom 
notations than appear on the finished printed charts. 
Such information was used for drawing the boun¬ 
daries between the different bottom types which ap- 
j)ear on the wreck charts for the Atlantic and Gulf 
Coasts, and for bottom sediment charts covering 
areas in the Philippine Islands. 

In the case of foreign areas, it is necessary to con¬ 
sult the archive files of the Hydrographic Office, not 
only for their latest edition of any particular chart, 
but also for all the foreign editions from which it was 
compiled. I’here is a tendency on the newer charts to 


RESTRICTED 










94 


COASTAL WATERS 



Fk;l'RF. 23. Pholograph of a rough, sandy bottom show¬ 
ing ripple marks caused by currents. 


omit bottom notations and frequently earlier surveys 
may furnish additional information. 

The British surveys of regions outside their own 
coastal waters are excellent, particularly in the Far 
East, and the French and Germans have also done 
good work off their more limited foreign possessions. 
The Japanese Hydrographic Office has published 
many detailed surveys of the Home Islands and the 
adjacent Asiatic coast. Data from these latter charts 
were carefully examined from a geological and topo¬ 
graphic point of view, and the bottom information 
w'as found to be in line with what might be expected. 
The charts compiled from them are believed to be 
fidlyas accurate as those compiled from other sources. 

The Archives of the British Admiralty have also 
been available and photostats of field data sheets as 
well as of earlier sinweys have been furnished for 
critical areas—material which was not available in 
Washington. 

Certain conditions must be borne in mind when 
charts of the bottom are drawn from information on 
the na\igation charts and not from actual material. 
If the notations on the charts were everywhere ac¬ 
cepted at their face \alue, many mistakes rvould re¬ 
sult. As stated before, the samples were taken as an 
adjunct of sounding with a lead, and a very small 



Fici're 24. Photograph of a coral bottom. 


amount of material was obtained. .Several misconcep¬ 
tions may arise as a result. First, there is a tendency to 
overemphasize the mud areas and make them appear 
more numerous and larger than they really are. Any 
bottom that feels at all soft or a sample which looks 
dark colored and oozy when wet is generally called 
mud by hydrographers. A mechanical analysis of the 
material, in many cases, would show this sediment to 
be sand and mud. Second, any bottom which feels 
hard to the leadsman and from which no sample is 
obtained, is apt to be labeled rocky or coral, depend¬ 
ing on the latitude. In most cases, this is correct, but 
stones or small coral fragments, in some cases, would 
have the same “feel.” Third, as the sampling appa¬ 
ratus can pick nji only a small amount of material 
consisting of the smaller particles, only the gravel 
and sand fractions would l)e brought to the surface 
o\er some stony bottoms. Adequate dredge samples 
and also photographs have shown that gravel bot¬ 
toms usually have numerous stones. Consequently, 
except in glacial areas, it was decided to classify all 
gra\el bottoms as stony. The same applies to such 
classifications as pebbles and shells, which usually 
occur in conjunction with stones. Fourth, the term 
clay suggests that the bottom has been somewhat 
compacted so that it will reflect sound better than 


BE.STRIC I ED 


-4 







BOTTOM STRUCTURE 


95 


ordinary mud but probably not as well as a sand bot¬ 
tom. Since sand and mud is also an intermediate clas¬ 
sification, it is used for clay, d'hcn there are cases in 
which two or more bottom symbols are given, such as 
rock and sand. I'he acoustical significance of the rock 
portion is greater than that of the sand, and the sedi¬ 
ment is so classified. 

These, then, are the principles used in construct¬ 
ing bottom sediment charts from information that 
admittedly was not so adecjuatc as might have been 
desired. No chart is ever perfect, and the navigation 
charts themselves are subject to constant improve¬ 
ment. The bottom sediment charts depend on the 
hydrographic sur\ ey which jn eceded them, together 
with whatever geological information can be brought 


to bear. Obviously, a better bottom sediment chart 
can be constructed from a detailed, large-scale survey 
of an important approach than from a region where 
the soundings are widely scattered. Neither one is so 
accurate as a chart made from an area from which 
bottom samples and photographs have been ob¬ 
tained. Also it is probably true that at present the 
bottom sediment charts are more accurate from the 
geological standpoint than they are as predictions of 
echo ranging conditions. If greater accuracy is to be 
attained in predictions for foreign areas, further 
work in underwater sound should be carried out over 
all types of bottom where it is possible to take samples 
and photographs, so that all the unknowns may be 
investigated simultaneously. 






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f 


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I 





diSttitaiffltfiiL 


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I 



GLOSSARY 


AMBIENT NOISE. Noise present in the medium other than 
target and own-ship noise. 

BOl’RDON TL BE. A flattened curved tube which tends to 
straighten out under internal pressure, used as the driving 
element in pressure temperature gauges. 

BE. Bathythermograph. 

BUOYANCY. .As used in this volume, the net buoyancy, or the 
diflerence between the weight of the water displaced by a 
vessel and the weight of the vessel. 

CONA’ECd ION CEEE. .A vertical section of the isothermal 
mixed layer bounded by cool currents descending from a 
convergence and warm cuirents rising to a divergence. 

DENSITY GR.ADIENT. Change of density with depth. 

1SOB.ALI..AST I.INES. .A set of lines, on the SB I chart, each 
starting from a set of selected points on the temperature scale 
and passing through all points for which the net change in 
buoyancy, resulting from changes in water temperature and 
depth is zero for a submarine of a given compression. 

ISOH.AEINES. Lines drawn through all points having the same 
saliTiity. 

ISO I HERMS. Lines drawn through all points having the same 
temperature. 

L.AAT.R DEBl H. The depth of the mixed surface layer. 

L.AYER ETEECT. Reduction in the echo and listening ranges 
on a submarine located within or beneath a thermocline. 

.MIXED L.AA'ER. 1 he isothermal layer occurring at the water 
su rface. 

RE\'ERBER.ATION. Sound scattered dilFusely back toward 
the .source, principally from the surface or bottom and from 
small scattering sources in the medium such as bubbles of air 
and suspended solid matter. 


REVERSING I HERMOMETER. A deep-sea recording ther¬ 
mometer; the temperature reading at the desired depth is 
preserved by overturning the instrument to break the mer¬ 
cury column. 

S.ALINITY. Number of grams of salt jjer thousand grams of sea 
water, usually expressed in parts per thousand. 

S. ALINITY GR.ADIENT. Change in salinity with depth, ex¬ 

pressed in parts per thousand per foot. 

SBT. Submarine bathythermograph. 

SH.ADOW' ZONE. Region in which refraction effects cause ex¬ 
clusion of echo-ranging signals. 

SINKING CENTERS. Subarctic regions where strongly saline 
surface water of tropical origin sinks through the colder, but 
less saline underlying layers. 

.ST.ABILITY. 1 he resistance to overturn or mixing of the water 
column, resulting from the presence of a density gradient. 

•S'E.ATION. .A set of pairs of thermometer readings taken in a 
s|)ecific location at approximately 100-meter depth intervals, 
along with a corresponding set of water samples analyzed for 
salinity. .A station is “occupied” when these readings are 
procured. 

T. ARGET -ASPECT. Orientation of the target as seen from 
own-ship. 

TEMPERATURE GRADIENT. Change of temperature with 
depth, e.xpressed in degrees F per foot. 

d HERMOCLINE. .A layer of water in which temperature de¬ 
creases with depth; a negative temperature gradient. 

UPWELLING. Rise of the main thermocline, and the underly¬ 
ing deep-water mass, toward the upper levels as the warm 
surface water is swejJt away by the winds. 

\'ELOCITY GR.ADIENT. Rate of increase in the velocity of 
sound with increasing water depth. 


RES l RIC I E] 


97 






BIBLIOGRAPHY 


Nmnbeis such as Div. 6-501.11-M4 indicate that the dociuneiit listed has been niicrofilined and that its title apjjears in the 
microfilm index printed in a separate volume. For access to the index volume and to the microfdm, consult the .\rniy or 
Navy agency listed on the reverse of the half-title page. 


Chapter 1 

1. The Oceans, H. li. .Sverdrup, M. \\'. |ohnson, and R. H. 
Fleming, Prentice Hall, Inc., New York, N. V., 1912. 

2. Biological Chemistry and Physics of Sea Water, H. \V. Har¬ 
vey, Macmillan Co.,’New York, N. Y., 1928. 

3. International .lsl)ects of Oceanography, F. ^\’. \anghan 
and others. National .\cademv of Sciences, 1937. 

1. .\ Detailed Study of the Surface La\ers of the Ocean in the 
Neighborhood of the Gtilf Stream with the .\id of Rapid 
.Measuring H\drographic Instruments,” .\. F. Spilhatis, 
Sears Foundatioti Journal of Marine Research, M)l. 3, 1910, 
p|). 51-75. 

5. Description and Operating Instructions for Model CXJC, 
OFMsr-31 (Subcontract 1), NDRC 6.1-sr31-1735, WHOI, 
1915. Div. 6-.501.11-M4 

Chapter 2 

1. Sound Transmission in Sea Water, C. O’D. Iselin and \V. M. 
Ewing, Report Gl /1184, WHOI, Feb. 1, 1941. 

Div. 6-510-Ml 

2. Calculation of Sound Ray Paths Using the Refraction Slide 
Rule, NavShips-913, 1943. 

3. Sound Beam Patterns in Sea M'ater, G. P. Woollard, NDRC 

6.1-sr31-1730, WHOI, Oct. 10,1914. Div. 6-510.11-M9 

4. The Sonic Ray Plotter, Leonard I. Schiff, NDRC 6.1-sr30- 
1741, Report 1’246, NS-140, UCDWR, .\ug. 8, 1944. 

Div. 6-510.11-M8 

5. Prediction of Sound Ranges from Bathythermograph Ob- 
sen’ations, NavShlps-943-C2, 1944. 

6. Use of Submarine Bathythermograph Obsenmtions, Nav- 
Ships-900, 069, 1945. 

7. Instruction Manual for the Use of the Undencater Camera, 

J. Lamar Worzel and \W. Maurice Ewing, NDRC 6.1-sr31- 
740, NO-lOO, WHOI, Mar. 16, 1943. Div. 6-501-M2 


Chapter 5 

1. Hydrographical Tables, .M. Kntidsen, 1901. 

2. The Prediction of Afternoon Effect, C. O'D. Iselin and A. H. 

Woodcock, OEMsr-31, OSRD 845, NDRC Cl-sr31-137, 
WHOI, July 2.5, 1912. Div. 6-510.I-M2 

3. “Suiface Motion of Water Induced by Wind,” I. Langmuir, 
Science, No. 87, 1938. pp. 119-123. 

4. 4 heor\ of Surface Water Motion Deduced from the 
Motion of the Physalia,” .\. H. Woodcock, Sears Foundation 
Journal of Marine Research, \'ol. 5, 1944, pp. 196-205. 


Chapter 7 

1. The Oceans, H. f. Sverdrup, M. W. Johnson, and R. H. 
Eleming, Prentice Hall, Inc., New York, N. Y., 1942. 


Chapter 8 

1. An Acoustic Interferometer for the Measurement of Sound 

Velocity in the Ocean, R. J. Ibick, Report s-18, NRSL, 
.Sei)t. 18, 1914. Div. 0-510.22-M6 

2. “Surface Cooling and Streaming in Shallow Fresh and .Salt 
W ater,” A. H. Woodcock, Journal of Marine Research, \'ol. 
4, 1941, pp. 153-161. 

3. Measurements of the Horizontal Thermal Structure of the 

Ocean, Norman J. Holter, Report S-17, NRSL, .Yug. 18, 
1944. Div. 6-540.4-Ml 

Chapter 9 

1. A Review of the Problem of Predicting Ranges in Shalloiu 
JVater, George P. 4\'oollard, 4VHOI, Aug. 17, 1944. 

Div. 6-570.1-M4 


SUPPLEMENTARY 


Bathythermograph 

Manual for Bathythermograph Pilot Instructors, Report M250, 
OEMsr-30, UCDWR, September 1944. Div. 6-.50L12-M3 

Preliminary Studies for the Development of a Salinity-Corrected 
Bathythermograph, A. W. Jticobson, OEMsr-31 (Sidtcontract 
1), Bristol Co., July 25, 1944. Div. 6-501.1-Ml 

Prediction of Echo Ranges from Submarine Bathythermograph 
Obserx’ations. Instruction Manual for Submarine Bathyther¬ 
mograph Observers (Part II), Richard H. Eleming and others, 
WHOI, UCDAVR, and Buships, Sept. 1, 1942. 

Div. 6-501.11-Ml 


Final Report on Project NO-221, Spectograms of Underwater 
Sounds, W. Koenig, Jr. and J. 4V. Emling, NDRC 6.1-sr346- 
1682, B EL, June 10, 1944. Div. 6-580-M2 

Current Methods for Prediction of Maximum SoiDid Ranges, 
Technical Memorandum 1, CUDWR, May 1, 1944. 

Div. 6-570.1-M2 

Sonar Tests of Tilting Beam Sound Gear on the USCCjiH5, U. S. 
Navy Oceanography Office, Mar. 4, 1944. Div. 6-570.21-M4 

Sotiar Echo-Ranging Tests on YMS370, Gordon .\. Riley, U. S. 
Navy Oceanography Office, Mar. 18, 1944. Div. 6-570.21-M5 


RESTRICTED 


99 






100 


BIBLIOGRAPHY 


Sutiar, Sub Riiits by USCGC Blanco, W'illiam Wood, U. S. Na\y 
Oceanogra})hy Olficc, May 15, 1914. Div. 6-570.21-M6 

Bathytberniograph Predictions, Key Ties/ Area, Gordon Riley, 
U. S. Navy Oteanography Office, June 7, 1941. 

Div. 6-580.1-M3 

Echo Range Data for the Period June 2 to June 15, 1914, 
Gordon Riley, IT. s. Navy Oceanographv Office, July 3, 1944. 

Div. 6-570.21-.M8 

Eeho-Ranging Data, Miami Area, L. V. Worthington and Helen 
S. Magee, U. S. Navy Oceanography Office, Jidy 3, 1944. 

Div. 6-570.21-M9 

Eeho-Rafiging Tests on the DD745, Gordon A. Riley, II. .S. Navy 
Oceanography Office, July 20-23, 1944. Div. 6-570.21-M7 

Eeho Range Data, Key TlWt Operating Area for the period 
June 16 to Oetoher 13, 1911, Got don A. Riley, 1 honia.s S. 
Austin, and others, U. .S. xNavy Oceanography Office, Jan. 17, 
1945. ^ ' Div. 6-570.21-5112 

W'oods Hole Oceanographic Institution Ileports 

Ax’erage Summer Sound Ranging Conditions in the Mediter¬ 
ranean Sea, C. O D. Iselin, Apr. 13, 1942. Div. 6-501.2-Ml 

Achieved Eeho Ranges of Surfaee Ship Targets in Deep and 
Shalloiv Water, Aug. 30, 1944. Div. 6-570.21-Mll 

Aeoustic Properties of Mud Bottoms, George P. AV’oollard, 
Dec. 6, 1944. Div. 6-510.5-M4 

Leeture Notes on the Use of the Submarine Bathythermograph, 
July 1945. Div. 6-501.11-M6 

A Comparison of Aehieved and Predicted Eeho Ranges on Suh- 
merged Submarines in Deep ]]'ater, George P. W'oollard, 
Aug. 19, 1944. Div. 6-570.21-M10 

Echo Range Obsen>ations of the USS Sylph, Maurice Ewing, 
NDRC C4-sr31-142, Sept. 10, 1942. ’ Div. 6-570.21-512 

Explosion Sotinds in Shallme ]\'ater, 5Ianrice Ewing and 
J. Lamar Worzel, Oct. 11, 1944. Div. 6-510.23-5112 

Echo Ranging Characteristics of Cape May, Nexe Jersey Area, 
Richard .5. Geyer and H. T. Bauerle, Jr., Buships Report 10 
from 5VHOI, Sept. 8, 1945. 

Factors Affecting Long Distance Sound Transmission in Sea 
Water, a Study of Radio Aeoustic Ranges from Data Taken 
by the U. S. Coast and Geodetic Sun’cy, OSRD 1505, NDRC 
6d-sr31-426, 5Iar. 30. 1943. ’ Div. 6-510.1-511 

Laboratory Studies of the Aeoustic Properties of iVakes, Part I, 
Seatteri)ig and Absorption of Sound by the Wake of Sound 
Ranges Under the Sea, Thomas H. Osgood, NDRC C4-sr20- 
100,"CCDWR, June 5, 1942. " Div. 6-500-511 

M iscellaueous 

Use of Bottom Sediment Charts, NDRC, 5Iay 1943. 

Aeoustie Properties of Gas Bubbles in a Liquid, Lyman Spitzer, 
OSRD 1705, NDRC 6.1-sr20-918, UCD4VR, July 15, 1943. 

Div. 6-540.22-511 

Change of Average Peak Eeho Intensity with Changing Ping 
Length, Lyman Spitzer, CUDWR, 5Iar. 22, 1945. 

Div. 6-530.1-514 


Comments on: Dissipation of Energy Due to Presenee of Air 
Bubbles in the Sea, Conyers Herring. Div. 6-540.2-513 

The Extinction of Sound in Water, Carl Eckart, Lh S. Navy 
Radio and .Sotind Laboratory, File 01.70, NDRC C4-sr30-021, 
IICD5VR, Aug. 31, 1941. Div. 6-510.11-Ml 

Fluctuation of Transmitted Sound in the Oeean, 7'echnical 
5Iemorandum 6, NDRC 6.1-srll31-1883, CLIDWR, Jan. 17, 
1945. Div. 6-510.3-514 

Mierostructure of Sea iVater. Notes on Measuring Temperature 
Differences, Harry Nycpiist, C4-NDRC-0-45, 5Iar. 12, 1942. 

Div. 6-520.1-515 

Mierostrueture in Sea fVater. Theoretical Analysis, Harry Ny- 
quist, C4-NDRC-046, 5Iar. 17, 1942. Div. 6-520.1-516 

Notes on Mass Transport Velocity, H. LI. Sverdrup and 5Vh 
51unk, Scripps Institution of Oceanography. 

Div. 6-501-516 

Preliminary Measurenients on the Aeoustic Properties of Dis¬ 
turbed Water, Eginhard Dietze, NDRC C4-sr20-205, USRL, 
.Sept. 7, 1942. Div. 6-540.3-511 

The Propagation of Underwater Sound at Low Frequencies as 
a Function of the Acoustic Properties of the Bottom, John 
51. Ide, Ricliard F. Post, and William J. Fry, Report S-2113, 
xNRL, Aug. 15, 1943. Div. 6-510.5-511 

Visibility in Ocean iVater, F. A. Jenkins, 1. S. Bowen, and F. T. 
Rogers, Jr., NDRC C4-sr30-024, UCDWR, Oct. 18, 1941. 

Div. 6-501-511 

Laboratory Studies of the Aeoustic Properties of Wakes, Jeffries 
Wyman, Wendel Lehman, and David Barnes, Part 11, Role 
of Bubbles in Wakes, Jeffries Wyman, NS-141, NDRC 6.1- 
sr31-1069, 5VH01, 5Iarch 1944. Div. 6-540.3-513 

Long Range Sound Transmission. Interim Report 1, for the 
Period Mareh I, 1911 to January 20, 1915, 5Iaurice Ewing 
and J. Lamar Worzel, NOBs-2083, Aug. 25, 1945. 

Div. 6-510.1-514 

Memorandum Summarizing Diseussions with Lieut. Rather, 
Coneerning Possible Methods of Overeoming lery Bad Sound 
Conditions, WHOI, Dec. 12, 1945. " Div. 6-570.1-517 

Oeeanography of Subsurfaee ]]’arfare, Lecture by C. O’D. Iselin 
at New London, 5Iar. 14, 1944. Div. 6-501-513 

Oeeanographie Factors Entering Into Harbor Protection by 
Underwater Sonic Dei'ices, WHOI, Jan. 21, 1942. 

Div. 6-510.4-511 

A Preliminary Study of Echo-Ranging Conditions in the Casco 
Bay .Irea of the Gulf of Maine, Richard .5. Ge\er, WHOI, 
Fei). 17, 1945. Div.‘6-501.2-514 

Role of Bubbles in the .leoustie Properties of tVakes, Jeffries 
Wyman, NS-141, NDRC 6d-sr31-437, 5VH01, Feb, 18, 1943. 

Div. 6-540.3-512 

Tests Made leith OD.i /QK.l Stabilized Depth Determining 
Gear on the USS Sonnies to .Iseertain the Aecuraey of Depth 
Indications leith Adverse Temperature Conditions, Thomas 
S. Austin, WHOI, Jan. 11, 1946. Div. 6-570.21-5113 

.Innual Cyele of Temperature and Salinity Betieeen the Surfaee 
and a Depth of 150 Feet in the ]Vestern North .Itlantie, 
C. O'D. Iselin, WHOI, Feb. 15, 1944. Div. 6-501.2-513 


RESTRICTED 







BIBLIOGRAPHY 


101 


A Summary of Factors Gox'eruiiig Sound Ranges, Cieors^e P. 
Wooilaui, WHOl, June 19U. Div. 6-57()-MI 

Sou)id-ra)igiug Experiments at Key July, I9fl, Maurice 

Ewing, OSRD 725, NDRC Gl-srSl-130, WHOI, May 23, 1912. 

Div. 6-570.21-MI 

The Transparency and Color of Ocean ]\'alers xeitli Special 
Reference to the Western Pacific, George L. Clarke and Henry 
D. Russell, WHOI, May 23, 1944. Div. 6-501-M4 

Use of Surface Vessel Bathythermograph Obsemations, Buships, 
December 1943. Div. 6-501.12-Ml 

Theory of Propagation of Explosive Sound in Shallmv IVater, 
C. L. Pekeris, CXSRD 6545, NDRC 6.1-srl 131-1891, CCDWR, 
January 1945. Div. 6-510.12-M5 

Bathythermograph Readings Accompanying Sound Gear Tests 

on USCGJ35, Gordon A. Riley, U. S. Navy Oceanography 
Oflice, Feb. 24, 1944. Div. 6-.501.12-M2 

Statistical Importance of Oceanographic Factors in A S Attacks 
hy Surface Craft, Lyman .Spitzer, Jr., NDRC 6.1-srl 131-1140, 
CUDWR, Dec. 23, 1943. Div. 6-510.4-M4 


Oceanographic Studies, H. B. Hoff, J. J. Markham, and G. R. 
Perry, NDRC 6.1-srl 128 1933, Report P28;12I4, N.S-140, 
CUDWR-NLL, May 5, 1915. Div. 6-.50I-M5 

Bi-ieeekly Report Cox'ering Period July 2‘> to August 7, 19J3, 
NDRC 6.1-sr3I-7.53, WHOI, Aug. w', 1913, pp. 1-2. 

Div. 6-510.41-Ml 

Telegraph Theory. Electrical Eqiiix'alent of the Ear as a Re¬ 
ceiver, Harry Nycpiist, Jan. 13, 1942. Div. 6-.560.1-M1 

Binaural Phenomena, Ralph C. Maninger, Report P12/4089, 
ClID^VR-NLL, Sept. 29, 1942. Div. 6-560.1-M2 

Interval Tests, H. L. Rumbaugh, Report D13/249, CUDWR- 
NLL, Apr. 8, 1943. Div. 6-560.1-M3 

Tone Duration as a Factor in Pitch Discrimination, E. G. 
Wever, Report M-179, UCDWR, Feb. 16, 1944. 

Div. 6-560.1-M4 

.4 Study of Binaural Perception of the Direction of a Sound 
Source, Irving Langmuir, V. J. Schaefer, and others, OSRD 
4079, NDRC 6.1-sr323-1840, General Electric Co., June 30, 
1944. Div. 6-560.1-M5 


RE.STR1CTED 








CONTRACT NUMBERS, CONTRACTORS. AND SUBJECT OE CONTRAC TORS 


Contract 

Number 

Name and Address 
of Contractor 

Subject 

OEMsi-20 

The Trustees of Columbia University 
in the City of New York 

Studies and experimental investigations in connection 
with and for the development of equipment and 
methods |)ertaining to submarine warfare. 

OEMsr-1131 

The Trustees of Columbia University 
in the City of New York 

Conduct studies and investigations in connection with 
the evaluation of the applicability of data, methods, 
devices, and systems pertaining to submarine and 
subsurface warfare. 

OEMsr-31 

Woods Hole Oceanographic Institution 

W oods Hole, Mass. 

Studies and experimental investigations in connection 
with the structure of the superficial layer of the 
ocean and its e:ects on the transmi.ssion of sonic and 
supersonic vibrations. 

OEMsr-30 

The Regents of the University of 

California 

Berkeley, California 

Maintain and operate certain laboratories and conduct 
studies and experimental investigations in connec¬ 
tion with sulunarine and subsurface warfare. 

NDCrc-40 

^Voods Hole Oceanographic Institution 
Woods Hole, Mass. 

Studies and investigations in connection with the 
oceanographic factors influencing the transmi.ssion 
of sound in sea water. 

OEMsr-287 

President and Eellows of Harvard 

College 

Cambridge, Mass. 

Studies and ex|rerimental investigations in connection 
with (i) the development of equipment and devices 
relating to subsurface warfare. 

OEMsr-1046 

Massachusetts Institute of Technology 
Cambridge, Mass. 

Studies and experimental investigations in connection 
with (1) underwater sound transmission and bound¬ 
ary impedance measurements: (2) ship sound surveys 
at high fretjuencies; (3) development of devices for 
the control of underwater sounds; and (4) develop¬ 
ment of intense underwater sound sources for special 
purposes. 


lOiJ 


RESTRICTED 











SERVICE PROJECT NUMBERS 


The projects listed below were transmitted to the Executive Secretary, 
National Defense Research Committee, NDRC, from the War or Navy 
Department through either the \Var Department Liason Office for the 
NDRC or the Office of Research and Inventions (formerly the Coordi¬ 
nator of Research and Development), Navy Department. 


Sen’ice Project Niiniher 


Subject 


NS-140 
Exi. 

NS-140 

NS-141 

NS-308 


Acoustic properties of the sea bottom 

Range as function of oceanographic factors 
Acoustic properties of wakes 

Sonar-surface and submarine hatliythermograph instruction program 


RESTRICTED 


103 














INDEX 


The subject indexes of all STR volumes are combined in a master index printed in a separate volume. For access to the 
index volume consult the Army or Navy Agency listed on the reverse of the half-title page. 


.\bsorption by mud, acoustical, 15 
Absorption of light by ocean water, 33 
.\ssured range and layer depth chart, 12, 

32 

Bathythermograph; description of, 5, 8 
design criteria, 5 

effect of weather conditions, 38-39 
history of development, 4 
isoballast lines, 23 
salinity compensated, 8 
Bathythermograph card, sidmrarine, 23 
Bathythermograph observations, world 
distribution map, 12 
Beaufort scale of wind velocity, 12 
Bjerknes circulation theory, 66 
Bottom reverberation, 15, 20 
Bottom sediment charts, ocean, 14, 91 

Camera, underwater, 15, 93 
Charts; a.ssured range (sonar), 12, 32 
bathythermograph observations, 13 
layer depth, 12, 32 
ocean bottom sediments, 14, 94 
ocean currents, 68 
ocean temperature variation, 32 
jjeriscope depth range (sonar), 12 
salinity of ocean, 53 
sonar conditions, 12 
sonar performance prediction, 12 
wind directions over oceans, 69 
wrecks, 93 

Circulation, ocean; see Ocean currents 
Clouds, effect on ocean thermal gradi¬ 
ents, 40 

Coastal mixing of sea water, 78-89 
Convection patterns, ocean, 41 
Convergence of ocean water, 41,68 
Coriolis force, 59 

Currents, ocean; see Ocean currents 

Density of sea water, 27, 59, 66, 69 
Diurnal warming in the ocean, 34 
Divergence of ocean water, 68 
Diving techniques, submarine, 22-26 

Eddy formation in oceans, 64, 72 
Ekman spiral, 66 

Fish, sonar echoes from, 21 
Fluctuations in ocean, small-scale, 75 
Foucault pendidum, 60 


“Fresh pocket” in ocean, 71 

Georges Bank water conditions, 80 
German oceanographic research, 3 
Gradient current, 68 

Heat exchange in the ocean, 33-77 

Inertia circle, 61 

Internal waves in ocean, 73 

Isoballast lines on bath) thermograph, 

23 

Layer depth chart, oceanographic, 12, 32 
Layers in ocean, thermal, 10, 27-59 
Light penetration into ocean, 33 

Manuals, sonar range prediction, 10 
Maps, marine; see Charts 
Microstructure of ocean, 75 
Mixing of sea water, coastal, 84 
Mud, as an acoustical absorber, 15 

Negative thermal gradients in ocean, 38 

Ocean column stability, 28 
Ocean currents 

charts of world currents, 68 
Coriolis force, 59 
ilownward curients, 41, 68, 70 
ed(h formation, 64, 72 
effect of water density, 59 
effect of winds, 66, 69 
effect on sonar range, 70 
gradient, 68 
in coastal waters, 77 
local variability, 72 
mechanism of formation, 59-68 
relation to density structure of the 
water, 66, 69 
thermal, 28-29 
tidal, 80 

Ocean lluctuation, small-scale, 75 
Ocean layers, thermal, 10, 27-59 
Ocean temperature; see Temperature of 
the ocean 

Ocean water; density, 27, 66, 69 
salinity, 51-55, 77-91 
Oceanographic research, German, 3 

Periscope depth range chart, 12 
Propagation of sound in sea water, 17-21 


Radiation, effect on sea surface tempera¬ 
ture, 33 

Rain, effect on temperature of ocean, 33 
Range, sonar 

effect of internal waves, 75 
effect of ocean bottom structure, 91 
effect of ocean currents, 70 
effect of ocean microstructure, 75 
effect of refraction, 20, 36 
limiting factors on, 10 
prediction, 10, 12 
variation with time of day, 36 
Refraction, effect on sonar range, 20 
Refraction slide rule, 9 
Refraction theory, undertvater sound, 9 
Research institutions, oceanographic, 4 
Reverberation, bottom, 20 
Reversing thermometer, oceanographic, 
29 

River water mixing with the ocean, 91 

.Salinity of the ocean; charts, 53 
coastline effects, 77-91 
correlation with temperature, 51 
gradients, 19, 52, 78, 87 
Salinity-compensated bathythermo¬ 
graph, 8 

Sea water, salinity, 51-55, 77-91 
Seasonal thermocline, 44 
Seasonal variations in ocean temjjera- 
ture, 44-50 

Sediment charts, 14, 93 

Sediments, marine, 92 

Sinking centers in ocean, 70 

Skip distance effect, 20 

Slide rule, refraction, 9 

Sonar, false echoes, 21 

Sonar echoes from fish, 21 

Sonar range; see Range, sonar 

Sound absorption by mud, 15 

Sound propagation in sea water, 17-21 

Sound velocity in sea water, 19 

Stability of ocean water columns, 28 

Subarctic convergence, 29 

Submarine diving technicjiies, 22-26 

Submarine supplements, 13 

Temperature of the ocean; air-sea tem¬ 
perature difference,40 
charts, 32 

coastline effects, 77 

correlation tvith salinity, 51-55, 82-83 
depth-temperature curves, 30 


RE.SIRICIED 


105 




106 


INDEX 


diurnal variation, 34-40 
efTect of weather, 33, 40-44 
heat exchange relations, 33-44 
seasonal variations, 44-50 
thermal layers, distribution of pri¬ 
mary, 27-32 

Thermal circulation of ocean water, 
28-29 

Thermal gradients in ocean, 19, 27-32, 
38 


Thermocline, main, 27 
Thermocline, seasonal, 44 
Thermometer, oceanographic reversing, 
29 

1 idal currents, 80 

Transmission of sound in sea water, 
17-21 

Underwater camera, 15, 93 
Upwelling, 32, 78 


RESTRICTED 


\'elocit) of sound, variation in sea water, 
19 

“Wa ter-boltle”(oceanographic instr.), 29 
Waves, structure of ocean, 43 
W'ind direction charts, 69 
Wind, effect on ocean currents, 66, 69 
effect on ocean thermal layers, 42 
Wind velocity scales, 12 
\\4ntcr ocean layers, 28 
W'reck charts, 93 






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